I don't think that "neither control" is the cause of Overtime, or at least not the only one.
I think that if the numbers are close enough (say, within 20 "seconds" on the main timer?) but the team that is behind currently has control, it will go into overtime. The match will end if you regain control of the territory back, or if they take the lead during this overtime.
I had Overtime 3 times, and 2 of those times the match ended as soon as a team took the lead again, and the last time it was when we took control of the base.
I think it's so that if you "would've had a fighting chance if the timer hadn't run out" that you have that chance to prove that you can hold it long enough for a victory.