I think mismanagement of volunteers is a big factor.
If you take on a volunteer, you have to respect the fact that they are volunteers and make sure there's a good match between what they want and are able to do, and where you place them.
For example, I've never been able to fit my professional studies and work round volunteering with the Guide Association because every time I voulnteer, and explain my circumstances (i.e. unpredictable hours, not much time, but willing to help where I can), I without fail end up getting sent to a failing unit whose leader is leaving/has just left, who are desperate for someone to take over, and where there's no discipline and no structure, etc. Which I can't cope with due to aforementioned unpredicatable hours and not much time, and have to quit.
Or once you're in somewhere, the district is so over-stretched that they end up pressuring everybody to give more and more time, until it all gets too much.
Or they send you to do Guides when you wanted to do Brownies, or vice versa, because that's where they need the leaders.
I just phoned up our DC over Easter, thinking "this time I'm going to explain it properly and it's not going to happen", and I explained in great detail about my job and how not infrequently I have to work late without notice, and that I would have to ring up and say I couldn't make it, which she said was fine. She put me in touch with a local unit and I went up to visit. The scenario (surprise surprise) is this:
The leader is coming up to 65 and is going to retire, the woman who helps her wants to leave as well and, anyway, has no warrant. There is no immediate prospect of any more help, as the neighbouring unit has just folded due to no adult leaders. On arrivak I was introduced to the kids as the new Brown Owl who will be taking over the unit when current Brown Owl leaves. WTF?