Sloppy or extremely busy and/or over-worked. Throw a few desk side jobs at me while I'm maxed out with story points and I can practically feel the bugs flowing into the code. I'll get it all done but...someone is going to pay later.
I wrote code with no pressure, tested it used it for months, and with some input that I didn't think about it broke spectacularly... I might be below average programmer (although I would say no, from what I have seen so far), but as I said, the chain is just as strong as its weakest link.