Why is the Pali word 'saṅkhārā' in Dependent Origination plural?
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.