The first return penalty was a personal foul by Jenkins... he dove at the feet of two players - "blocking below the waist" - but it was, as you inferred, completely unnecessary.
Agreed, total bonehead play by Jenkins.  He did redeem himself though by throwing the last block to get Randolph into the endzone on the second return.  Then he takes his helmet off on the third PR for TD for another 15 yarder.  :
I like Mike, but he seems to make some really stupid plays.