pronunciation - Why is 輩 in 先輩 pronounced ぱい while 輩 in 後輩 is pronounced はい?

I wonder in this case if there is any rule that turns H to P.

Answer

TL;DR
In Japanese (native) and Sino-Japanese words, ん and っ make the next consonant //h// to //p//. The sound change is a grammatical demand that no longer has phonetic ground, as much as English critic vs criticize.

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Long story
Once upon a time, the Japanese consonant that we today call //h// had the sound [[p]]. This sound was weakened to [[ɸ]] by at latest 10th c. (母は昔はパパだった), then to [[h]]-like sounds (except ふ) by at latest 18th c. (ハマの二つは唇の軽重). But there are two cases that these changes were met with resistance.

1. 
   

   //p// preceded by consonantal element, now converged into either ん (nasal stop) or っ (reduplication)

   
   

   
   

   歩 [[po]] → [[ho]] "step" : 一歩 [[it.po]] → [[ippo]] "one step"
   歯 [[pa]] → [[ha]] : 出歯 [[de.ba]] "bucktoothed" (rendaku) : 出っ歯 [[deʔ.pa]] → [[deppa]] "bucktoothed (slangy)"
   輩 [[pai]] → [[hai]] "fellow" : 先輩 [[sen.pai]] → [[sempai]] "senior"

   
   

   
   
   


2. 
   

   mimetic words; that carry sound symbolism unlike usual words

   
   

   
   

   ぴかぴか [[pikapika]] < 光 [[pikaɾi]] → [[çikaɾi]] "light"
   ぱたぱた [[patapata]] < はたく [[pataku]] → [[hatakɯ]] "pat, rap"

   
   

   
   

From today's perspective, it seems that //h// occasionally turns into //p//, but it's actually //p// is occasionally retained until today.

PS
ヨーロッパ, the Japanese transcription of Portuguese Europa, attests two important facts at the Late Middle Japanese stage: (1) reduction of diphthongs //eu// → //joo//; (2) //p// unable to appear in the middle of word without reduplication *//-pa// → //-Qpa//.