Why is a plant flowering in my veg room?
In theory there are several reasons this could be happening but if you are certain that you do not have an auto flowering variety, and you are confident that your timers are working than there are a lot less likely reasons.
So first be sure that your lights are on for a min of 18 hours per day and only off for 6.
The next piece of advice would be to monitor the grow space, it is possible that one of the taller plants (if there are any) may be blocking this plants light, if that is not the case the lights in the room may not be close enough to the plants and if possible should be lowered.
Read more about how light affects the grow cycle.