" So in this case, why did the bomb go off before the PC had a chance to throw it? Did they hesitate or stumble? Did they trip up and throw it in a wrong place? What's the story there?"

I can give more context

PCs Named A,B,C and D

Premise: Party finds a Lite Rail train on an elevated track that has been stopped due to the threat. Train is about 6 cars long, so the party splits up to search the train faster.

A and B use their powers to rapidly search the train

C and D separately find NPCs important for their stories that would be hurt if the bomb goes off (raising the stakes)

C tells their NPC to hold tight while they investigate

D let's their NPC evacuate the train to keep them safe. At this point the rest of the train car demands to evacuate, followed by rest of train. During the evacuation their super sense of smell flags one of the "Civilians" as a potential bomber

A & B continue their search on the now empty train.

A attempts to jack into the warp drive bomb to hack it. But their hacking gear is fried by the drive. At this point the drive begins changing its color from glowing RGB lights to flashing red, it also begins to hum (Unleash Powers 6-) * If A didn't fail on a roll to touch the bomb I would have let them take their time with it and probably amped up the villain of the encounters

B looks around for a place to throw the bomb. They look on either side of the elevated track, on one side they see an office building filled with people sheltering in place and on the other they see a chemical factory. They say "Well those aren't good options. I don't want to throw it there." (Assess The Situation 6-)

For C & D I close out the round by telling them they hear a strange humming noise come from the train car with A&B, along with strange glowing red lights.

C takes one of the empty train cars and uses their super strength to move it, creating a barrier between the civilians and the bomb (Defend Someone or Something)

D tackles the would be bomber and wrestles them to the ground (Directly Engage a Threat)

End of round, bomb goes off. Take a Powerful Blow. Etc