As shown in the Wiktionary:
From Middle English plege, from Anglo-Norman plege, from Old French plege (Modern French pleige) from Medieval Latin plevium, plebium, from Medieval Latin plebire "to pledge", from Frankish *plegan (“to pledge, support, guarantee”), from Proto-Germanic *plegō (“responsibility, habit”), from Proto-Indo-European *dlegh-.
Akin to Old High German pflegan (“to take care of, be accustomed to”), Old Saxon plegan (“to vouch for”), Old English plēon (“to risk, endanger”). More at plight.
But I can't find the clue to this sound change on Wikipedia, which concludes that only PIE *b will become PGmc. p-.
PS: AHD contains the same etymon dlegh