I've been around enough blocks to recognise at least two separate issues here, and neither of them are younger-generation things.
The first two, the drummer and the student, both sound like they got overwhelmed beyond their ability to cope with the situation at all. The pattern I recognise here is somebody suddenly realising they're already in so far over their heads, or the psychological weight (as felt from the inside, before any kids-these-days types start yelling again) that they have no idea how to cope with it. Some freeze, as in the fight, flight or freeze thing, others don't know how to climb back down and feel that the shame of disappearing is preferable to the stress of facing up to the people that they're letting down. In the latter case, yes, they often know what doors they're closing, but as far as they're concerned it's now a question of how those doors are slammed on them, not whether.
I've seen this in normal, healthy people who simply didn't have the experience to know how big the thing would seem when they got there, but particularly in people with an anxiety disorder. If your baseline is somewhere between anxious and freaking out simply because your brain chemistry is out of whack for genetic reasons, getting overwhelmed is a regular fact of life.
The part about coming back and asking for a reference could be a few things, but mostly sounds like a critical lack of self-awareness - particularly common in easily-outraged old white men, but seen in all demographics.
The social media consultant could be a bunch of things: flake, got fired, lost interest and wandered off, personal crisis, or health emergency. Hell, she's a young woman - the chances are uncomfortably high that she's recovering from being raped.
I get (and believe me, have gotten) the frustration when somebody just stops responding, but the upshot is that there's a range of possible reasons, only one of which is the one you're implying: a deliberate, considered decision to walk away and cut ties.
So the best I can offer there is the suggestion to change tone when trying to re-establish contact, assume that there's a very real crisis on their end, and ask what you can do to help. Even if they're just being a flake, it's probably your best chance of getting clues about what's actually going on there. If you have acquaintances in common, try asking them what's wrong, instead of going direct to the person in question - if the person has a history of being a flake, common acquaintances are more likely to tell you that outright. Or, if it was something you said or did, they're also more likely to tell you than the person themself.
Ways of pre-empting this kind of thing? There isn't a silver bullet. You can learn to spot signs that somebody's likely to be less reliable than you need - mostly through painful experience, unfortunately. You can learn to ask questions that tell you how they've delivered (or not) in similar situations, which can also be a useful gauge.
You could find a way to "subtly" mention that somebody already let you down on a similar project, and lament that if they'd only reached out and told you what they were dealing with, you could have found a way to work around it, or at least know that you needed to go ahead and find somebody else - then, if some crisis blows up, there's a better-than-even chance that they'll take the chance of facing your wrath. Then it's on you to take a deep breath and not visit your wrath on them, to cultivate trust in them and in people they tell about how that situation worked out. Or they might not, but it does improve the odds.
The general issue here is that you've run into a Whiskey Tango Foxtrot, and you want to know what state of mind produced that.
It's a regular enough problem among the software developers in the company I work for, that we added "seek first to understand" to the existing set of company values. With large, long-lived codebases, it's common for people to see a section of code for the first time and think, "what kind of lunatic thought that was a good approach to this problem?! Holy cow, what a mess!" and the answer is often that it was actually a reasonable choice under the circumstances - it's just that you get further by asking the perpetrator what the circumstances were, than by venting at them.
Vent away if you need to, just not in a direction that's going to make it harder to resolve the problem at hand.
That was a mite longer than I thought it was going to be; apparently I had more on the subject than I realised at first.
