just a guess.

Because this ability is active, and the pop and awe upgrade will remove Bombardment Ability, but

This ability is in progress, so this upgrade can’t remove this ability, and this upgrade will add the Pop And Awe ability, so we get two ability.

And in normal cases, the mortar monkey shouldn’t have two ability, so the game is confusing, and the game don’t know which one ability should be active, so the game got a error, and the game is crash.

Edit: it was fixed

IT was fixed, yes