It has been there so long, unlikely TBS can fix it but hey might as well post it again just in the off chance they care, but I doubt it.
There is no rhyme or reason, just happens sometimes. Going to the other ballistae does not help, end of game.
I can guess I can add Claytons 3rd person video of the event - it happens often enough to be annoying
It is one of those bugs that should be fixed ASAP. It happens almost everytime.
It’s quite debilitating when it happens on maps such as citadel or hillside where the ballista is essential.