Why is the Pali word 'saṅkhārā' in Dependent Origination plural? In researching Dependent Origination as described in SN 12.2 and using a basic Pali language guide (page 18), I found the following about each main Pali word:


  jarāmaraṇaṃ singular from maraṇa neuter
  
  jati singular from jāti feminine
  
  bhavo singular from bhava masculine 
  
  upādānaṃ singular from upādāna neuter
  
  taṇhā singular from taṇhā feminine
  
  vedanā singular from vedanā feminine
  
  phasso singular from phassa masculine
  
  saḷāyatanaṃ singular from saḷāyatana neuter
  
  nāmarūpaṃ singular from nāmarūpa neuter
  
  viññāṇaṃ singular from viññāṇa neuter
  
  saṅkhārā plural from saṅkhāra masculine
  
  avijjā singular from avijjā feminine


The above seems to correlate with every translation I have read. Note: saḷāyatanaṃ may appear to be translated as plural but it appears singular because it includes 'six'. 

For example, all translations say: "What is feeling [singular]?" rather than "What are feelings [plural]?" 


  And what is feeling? Katamā ca, bhikkhave, vedanā? There are these six
  classes of feeling. Chayime, bhikkhave, vedanākāyā— Feeling born of
  contact through the eye, ear, nose, tongue, body, and mind.
  cakkhusamphassajā vedanā, sotasamphassajā vedanā, ghānasamphassajā
  vedanā, jivhāsamphassajā vedanā, kāyasamphassajā vedanā,
  manosamphassajā vedanā. This is called feeling. Ayaṃ vuccati,
  bhikkhave, vedanā.


The saṅkhārā condition is described as follows:


  And what are saṅkhārā [plural]? Katame ca, bhikkhave, saṅkhārā?
  
  There are three kinds of saṅkhārā. Tayome, bhikkhave,
  saṅkhārā— body saṅkhāro [singular], speech saṅkhāro [singular] and mind saṅkhāro
  [singular] kāyasaṅkhāro, vacīsaṅkhāro, cittasaṅkhāro.


Why is the Pali word 'saṅkhārā' in Dependent Origination plural where all other words are singular?
Dependent origination is the cycle of the happening event. Each event has only one decision. There are 2 saṅkhāra for each decision: wholesome and unwholesome. Wholesome saṅkhāra gives good resultants. Unwholesome saṅkhāra gives bad resultants. So, viññāna to vedanā which are the resultants of saṅkhāra must be singular, because it's just the resultants of one decision. Jāti and jarāmaraṇaṃ are same. Wholesome karmma-bhavo gives good resultants. Unwholesome karmma-bhavo gives bad resultants. So, jāti and jarāmaraṇaṃ must be singular because of the same reason.

Above explanation explained clearly in abhidhamma vibhaṅga paccayākāravibhaṅga and it's atthakathā. It separates each event to singular as "saṅkhāro". That's why no direct explaination in any atthakathā, included path of purification. However, it's used widely in difference context such as in VN.A. Introduction.

And this is the example of what I wrote "it's used widely":


  Kusalattika: kusalā dhammā, akusalā dhammā, abyākatā dhammā.
  
  The "wholesome" triad: wholesome phenomena, unwholesome phenomena, not
  both phenomena


The Path of Purification detailedly exposited the dependent origination follow to 'the "wholesome" triad':


  jarāmaraṇaṃ = not both phenomena
  
  jati =  not both phenomena
  
  bhavo =  not both phenomena
  
  upādānaṃ = unwholesome phenomena
  
  taṇhā =  unwholesome phenomena
  
  vedanā =  not both phenomena
  
  phasso = not both phenomena
  
  saḷāyatanaṃ =  not both phenomena
  
  nāmarūpaṃ = not both phenomena
  
  viññāṇaṃ = not both phenomena
  
  saṅkhārā = 1. wholesome phenomena, 2. unwholesome phenomena
  
  avijjā = unwholesome phenomena


You can see whether saṅkhārā are separated into 2 events.

Above quote is my own conclusion. To read it, the reader needs to understand in advance of chapter XIV, XV, XVI, XVII, and some part of chapter I, III. Also, it requires Abhidhamma knowledge in advance, as well.

Path of Purification, CH. XVII, THE SOIL OF UNDERSTANDING—CONCLUSION, DEPENDENT ORIGINATION, III. Detailed Exposition page 547-608.