Just curious, what can happen that delays the decision making process after an interview?

I had an interview Monday last week and they said they'd have a decision by Wednesday. I didn't hear anything so I kinda wrote it off, but the recruiter emailed me saying there was a delay and they hope to have everything finalized by next Monday.

I understand the day they say they'll finalize is only two weeks after the interview so not a big deal in the grand scheme of things. But in general, what causes a delay?

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