This is based on the following: You've told Mordon about Dalatrass Linron's offer, he's aware of the sabotage, Eve is alive as you kept Maelon's data and Wrex is leading the Krogan.
The lightning appears to be a secondary effect of the damage caused by the Reaper during your approach (where you're avoiding the reaper beam) to the Shroud Tower, and the Thresher Maw attacking the Reaper in very close proximity to the tower - see this video:
The lightning is not visible at long range both in the conversation before the approach and the conversation during the approach - if you look at around 6:50 there is a full upwards pan showing the tower with no lightning effect at all - yet if you look at 7:22 the tower has multiple minor explosions and lightning surrounding it.
As the long range shots also don't show explosions the only plausible conclusion is that the lightning is a secondary effect of severe damage to the shroud tower, which has also resulted in localised explosions.
The Shroud Tower is visibly falling to pieces as Shepard approaches. Mordin, in his personal desire to undo the actions of his past, personally wished to release the cure.
As he said himself; “Had to be me. Someone else might have gotten it wrong” - and due to the STG sabotage this could only be done from inside the tower's control room.
While he is releasing the cure, the Shroud Tower explodes and Mordin dies inside.