Is there a Buddhist explanation for why envy manifests as hatred? I have observed that when I am envious of somebody, I feel an impulse to malign them, and sometimes experience feelings of hostility toward them. Is there a textual explanation for this phenomenon?
You specified pali-canon but I think (admitting I'm merely skimming translations) that definitions of envy are similar in Mahanaya texts too; for example one of the Tibetan texts, A Necklace for Those
of
Clear Awareness
Clearly Revealing the Modes of
Minds and Mental Factors, defines it as,


  Jealousy (phrag dog)  
  
  Regarding jealousy, the Compendium of Knowledge says:
  
  
    QUESTION: What is jealousy?
    RESPONSE: It is a deep disturbance of mind that cannot tolerate another's
    marvelous attributes due to excessive attachment to gain and honor, and is
    involved with hatred. It has the function of [causing] mental unhappiness
    and of not abiding in contact [with happiness].
  
  
  Just as it has been said above, it is a deep disturbance of mind that cannot bear
  another's success due to attachment to gain and honor. It produces serious undesirable
  [consequences] in both this and future worlds [i.e., rebirths]; in this life there will be
  mental unhappiness, and in future lives one will be impelled into bad migrations.




On Access to Insight there's some textual explanation here, not in a sutta but in the translator's introduction to a sutta:


  A glance through the list (see Note 2) will show that all these sixteen defilements derive from greediness and selfishness, from aversion, self-assertion and conceit, or their combinations. If we take, for instance, contempt, being a weaker nuance of (5) denigration, we see that aversion and conceit contribute to it; (7) envy is fed by greediness and aversion. The pairs of contributive factors here exemplified do not, of course, occur at the same moment of consciousness; but their repeated, separate presence favors the arising of such derivatives as contempt and envy. On the other hand, if those secondary defilements such as contempt and envy (and all the others) appear frequently, they will bring about a close serial association of their "feeders," as for instance hate motivated by conceit, or hate motivated by greed; and these may easily become habitual sequences, automatic chain reactions in our impulsive life.
  
  Interlocked in such a manner, the negative forces in our mind — the defilements, roots of evil, and fetters — will become more powerful and much more difficult to dislodge. They will form "closed systems" hard to penetrate, [etc.]


I read that as saying that:


Envy is fed by greed and aversion
Hate is (or may be) motivated by envy
It becomes tangled, inter-related, hard to disentangle


The statement that "[factors] do not, of course, occur at the same moment of consciousness" is I'd guess an abhidhamma statement (that you can only be conscious of one thing at a time).



My personal opinion is that in theory I can imagine two motives for the impulse to malign someone:


A primitive belief in the zero-sum nature of things, e.g. "He has what I want, if I could destroy him then I could take his place and have that for myself"
Or perhaps some egalitarian belief, e.g. "If he has more than others, he doesn't deserve it"




Perhaps I'm wrong, it seems to me that the suttas don't examine negative emotions in great detail. Perhaps it assumes we know what they are, but doesn't dwell on them (and doesn't encourage us to dwell on them) -- for example, consider verses 3 through 5 of the Dhammapada.

I think the antidote for envy may be mudita, by the way, if you want to read more about that -- if you do read more about mudita then I think you'd incidentally read something more about the nature of envy.