I have been on the receiving end of being abandoned (or very much downgraded) and also backed out of a couple of difficult friendships.
Most of my friends stepped forward when things got horrendous in my life; a few stepped back. I was quite surprised and very upset for a couple of years, but now I see that these things ebb and flow. It also means that when things have gone wrong for them, which I don't wish on anyone, I don't need to step forward for them. I know now that it's not reciprocal, and that I can just send a Happy New Year text once a year or like their posts on FB, and I don't need to do any more than that.
Most of my friends were great though, helped with food, appointments and were just there for me. It was a long time though and I hated being the object of pity; it also makes you very boring as you pretty much only have stress and hard things to talk about, it's hard to remain upbeat when you are having the worst time, even if you are, there's a shadow over you.
I have backed out of a couple of friendships when it became clear their behaviour was just dysfunctional, as in over the top, dramatic and was sucking me in. I feel bad, I know they will have interpreted it as rejection but I can't risk my own MH in that type of way.
Mostly now I'm older I have a good sense of different types of friendship, and I wouldn't ask a 'out with the girls' type friend for favours or a friend I see once every couple of years for regular help. True friends who would drop everything are quite rare, I wouldn't do that for many and many wouldn't for me.
I think the risk is if you only have one or two friends, and you perhaps don't have a good reading on the nature of the friendship, you can be surprised when they back out, but I bet there were signs they weren't so invested before that, or that they liked a particular dynamic which is now spoiled.