Finally cracked and attempted to write the definitive friendly-but-firm response to random people on your open-source project asking "please assign me this issue". If you're in the same boat as an OSS maintainer, feel free to steal...

https://github.com/wagtail/wagtail/wiki/Why-you-shouldn't-ask-to-be-assigned-an-issue

@gasman I don't recall seeing this problem in the projects I'm involved in, but I do like your explanation
@bwh We do have a history of actively seeking out new contributors through programmes like Google Summer Of Code, so it may well be a side-effect of that.