Just at a guess, perhaps you looked over the fence when you could have satisficed instead, and then you got tangled in details rather than nailing just the main issues and then driving a bargain for more time?
That happens a lot in tech, especially if you frame the problem as one for which the solution must also demonstrate your competence. And more especially if you are generally an externally-referencing person, i.e. "how is this type of thing done" vs. "what can I see needs to be done here and what's the easy fix that comes to mind that I'd bet on" or a contingency planner, i.e. "what general perspectives or anti-patterns am I not aware of that will break this later" vs. "later will come later".
Still, not knowing all the details it's just a guess. Perhaps there's also blame to be placed on the organizational psychology and it's a lesson to push back more next time, or a mix of things...
