Seems like a developer that is getting literally nothing for his app or plugin is more likely to sell it than a developer that's getting some income from it. At the least, the buyer would need to match the current value that the app provides its author.
I mean, everyone's gotta eat. I think there's plenty of instances of someone building a tool that they personally find useful and then making that tool available for free, unsure of what the reception will be and not expecting a lot of users. But if that something does very well and finds a wide audience I think it's natural to try to earn a living from it. And, if the attempts to "monetize" fail (as they often do) and someone is offering a lot of money in a lump sum to take it off their hands, well frankly I think they'd be foolish not to take it. And if that arrangement happens to turn out poorly for the userbase, well hopefully that will be another small object lesson in paying for things you find useful, when politely asked.
(And yes, I'm aware that's a lesson that really should have been learned by now, if it was going to be learned at all. Alas.)
I think it starts with passion - he created some useful software, he shares it and initially enjoy working on it. Then he puts a donate link (I saw there was a donate link in the previous site), and gets almost nothing, but he still needs to add bug fixes, maybe new features, answer to the user's emails, etc.
After a while it becomes a chore... and still getting $0 out of it. And that's when he might want to find other ways to get something out of the efforts he put in. Unfortunately the only option is to turn it into malware since nobody wants to pay for it, or turn it into a decent profitable business.
I don't have a link handy but I distinctly remember Take Two, a giant corporation with billions in revenue, saying when they removed it from Kerbal Space Program that they promised to wait a while and be more subtle next time they did that.
