There can be two reasons for that. Either the enemy got close enough to trigger the explosion - and then you ran away. The explosion takes like a second, so you can still dodge it.
Or it was a chain reaction. One robot exploded and dealt damage to the others. So that's not a bug, it was designed this way :)
(I know, I know. If I had more time, I would make a proper animation for that. But you know, 3 hours is not very long :P)