So they key worked before you popped the keycap off? The logical conclusion would be that you damaged the switch when you popped it off, although it should really be quite difficult to damage a switch like that.
If you are confident you were gentle and didn't damage anything, maybe something occurred at the factory that caused a faulty switch + a faulty stabilizer at the same time (e.g., too forceful an application of the keycap, but this also seems unlikely).
I think #1 is more likely, especially if the key was working previously, but I wasn't there so can't say with 100% certainty.