Could it be that people who use your OSS think you have an obligation to them, because you marketed your software in a nice and pretty gift wrap package (eg. web page) and they chose yours over and alternative (ie chose to invest their time and effort on it)? They chose to "fall" for your marketing over that of another project.

Maybe they feel that by choosing your software over an alternative, they're also helping you (since it's your project after all) and they atypically expect some things of you?

P.S. I'm not taking sides. This is just a shower thought.