I'm not clear on the details of why, but I know if the fast isn't properly broken, then the body can react very poorly. I suppose it needs time to get back in the habit of processing food. The main thing to remember about a fast is that it will take as long as you fasted to break it (unless it's 24 hours or less). So if you water fast for 3 days, you'll need another 3 days to ramp back up to normal eating. If you juice fast, then it's 1/3rd of the time (ie, 1 day for a 3-day juice fast).

I've read about people ending up with allergic reactions, swollen joints, pain, and many other issues from improperly breaking the fast. Also, me not remembering or being clear on the details doesn't mean they aren't clearly laid out on sites/articles about this topic. That is, you can search online for a bit, and it should clear things up for you.

I'm not a doctor, but see https://en.wikipedia.org/wiki/Refeeding_syndrome

I don't know any of the science behind it, but it's my understanding that during long periods of not eating, metabolism gradually slows to a crawl to allow maximum duration from the amount of 'energy' available in the body.

I'd wager it gradually ramps back up again once food is reintroduced, and overeating during this period could probably lead to some pretty hefty weight gain.