What are the arguments behind the relatively late dating of Khuddaka Nikaya and Abhidhamma Pitaka? It is generally told that the Khuddaka Nikaya is the latest among the five Nikayas of the Sutta Pitaka. Similary, the Abhidhamma Pitaka is also considered as the latest composition among the three Pitakas. It seems that these are universally accepted truths among the scholars of Buddhism. But what are the exact arguments behind these two assertions?
For the layer of pāli-canons timeframe, please see:
What are timeframe were pali canons, included commentaries, done, especially Mahavihara-Theravada?
The arguments began from misunderstanding of students/scholars in reading study system, especially from the western world, see  The Case of Dhammakaya (Thai: กรณีธรรมกาย) by P.A. Payutto.
Reading study system make students/scholars understand teaching discontinuous, incomplete, through the buddhist study course.
You can see through below cases that the arguments of buddhist hypothesis by the students/scholars in reading study system, began from misunderstanding.
So, there are many misunderstand appearing from the past and still going on now, below.
I have the evidences in pāli-canons for each below example, if someone request to me, I will add them to this answer as soon as I can.
for the example:
(a) The introduction of dhammasaṅgaṇī's commentary, by the tipitaka-memorizers, said:

sāriputta studied just the brief abhidhamma teaching from buddha. Then
sāriputta taught his advance literary abhidhamma to his large number
of  students.
(Sāriputta's student was included upāli because some paṭṭhāna-words appear in
Vinayapitaka Parivāra, and some abhidhammas' description appear
in vinaya, too. So, in gosiṅgasāla-sutta said many famous
bhikkhu go to listened sāriputta's teaching, and in A.N. Ekaka. buddha
announced sāripuuta is the best of teacher who can teach the listener
for enlightenment as alike as himself. But sāriputta's literary abhidhamma didn't appear directly in 4 first nikāya of suttanta because even though ānanda often went to listen sāriputta, but he was not sāriputta's close student. When the 1st saṅgāyanā began, the most of sāriputta's teaching, still kept by sāriputta's students, not by ānanda. So, the fact appeared in commentary which said that many canon in K.N. were taught by sāriputta. Another, K.N. and Abhidhamma are easier to memorize than suttanta, by technical, too. So, the very big amount of abhidhamma and K.N. in oral study sysytem are not an argument to said &quot;abhidhamma began very late after buddha time&quot;.)

But the beginner students just read through commentary then misunderstood and distort it as &quot;commentary said buddha taught abhidhamma&quot;.

(b) The introduction of kathāvatthu's commentary, by the tipitaka-memorizers, said:

The forth member, of sevent abhidhamma canons, were authored in 3rd saṅgāyanā.

But the beginner students just read through commentary then misunderstood and distort it as &quot;abhidhamma authored in 3rd saṅgāyanā&quot;.

(c) In VN Pañcasatikakhandhaka, 1st saṅgāyanā saṅgha, the original commentary teachers, made the unanimous decision to keep, to modify not/to add not, all the buddha's speech rules in the Vinaya, even the lesser and the minor rules. If there are something to describe about vinaya, it must appear in the other's context, such as the upāli's sikkhāpadavibhaṅga/upāli's parivāra/commentary/etc., without any modify buddha's speech.
But the beginner students just read through some sutta, then misunderstood and distort it as &quot;there were just 150 vinaya rules in vinaya pitaka&quot;.

(d) Commentary said in introduction of D.N.'s  commentary:

Ānanda showed to kassapa, at 1st saṅgāyanā, that he did not modify anything of every sutta which he was presenting  to 1st saṅgāyanā saṅgha.

But the beginner students just read through the english translation version of 4 nikāya, which translated only some part, then misunderstood and distort it as &quot;some sutta in 4 nikāya maybe authored after 1st saṅgāyanā&quot;.

(e) Vinaya pitaka and DN's commentary, the tipitaka-memorizers, said:

1st saṅgāyanā saṅgha gave each nikāya and commentary to 5
arahanta-groups at 1st saṅgāyanā meeting cave, upāli school got
vinaya-pitaka, ānanda school got dīghanikāya, etc, to recite for
the received nikāya, and there were somebodies in each school memorized more than 1 nikāya, too. Then about each 100 years later their relayed
students met each other school members again to recite them together,
2nd and 3rd saṅgāyanā, again. Each saṅgāyanā, they did it on the same
rule in (c) and (d).

But the beginner students just read through the western scholars' thesis, who never recite and memorize tipitaka, then misunderstood and distort it as &quot;saṅgāyanā was the meeting to edit tipitaka.&quot; And &quot;tipitaka become blurred through the time, because of oral study system&quot;.

You can see through above cases that the arguments of buddhist hypothesis by the students/scholars in reading study system, began from misunderstanding.