As you say there would not have been a green light on the crossing however the lights ceasing to flash and the barrier going up would normally indicate that it was OK to cross A mental green light if you wish.
Normally there is quite a bit of time between the barrier coming down and a train crossing. It sounds as though the track circuit failed to register for some reason or there is a sequence failure which allowed the barriers to be momentarily raised after the south bound train had passed but did not keep them down because the north bound train was arriving at the same time.
I was always under the impression that the barrier sequence should start at at least 60 seconds before the train passes at line speed. That would mean that the train should have been at least 2 miles away when it triggered the sequence. As John describes it it may have been less than a mile away when the sequence was triggered.
Probably all BS and no doubt Bob will correct me in a moment.