okay a fair number of responses so far. However none of the replies seem to fit the situation i saw.
I shall elaborate on what i saw.
We kill the third bomber with 5ish seconds to spare, bomb carrier goes up and gets pulled back, burn commences. Walker is somewhere between 30% and 20% at start of this final burn. Kephess comes down tank picks him up. About halfway through the burn the additional bomber pops up and 2-3 shots the healer he picks. Total time between the kill of previous bomber and the new one somewhere in the 20-40 sec range.
Now i know the bombers come out pretty quick at the end if you miss one entirely but we are getting the bombers pretty quickly unless its the way far away door for the spawn even then it doesn't time out usually.
This happened to us this week with one notable exception: we didn't wipe, because the second tank picked up the extra bomber, pulled him off to a corner, and let him explode. Trivially dealt with.