Archimonde didn't blow up, that was the energy of the Tree/Wisp getting detonated at once.
What if Mannaroth blew himself on purpose? He was mortally wounded, it's possible he decided to simply take Grom with him so he couldn't have a chance to redeem his race. It makes it a lot more poetic actually. He blows himself up so Grom can die with the Demon Blood in him and as a disgrace, but Grom manages to survive long enough to die pure, and redeemed.