A crowd of mostly young or hungry entrepreneurial hackers who could clone his software and/or enter the same niche markets, perhaps taking only a few good sprints to do it, as well. I can definitely see why someone might be scared of this crowd.

I have a cash-cow side project that makes more money than many would expect. It'd also be trivially easy to duplicate and is simple enough that there'd be barely any value to HNers to hear the details. I'm confident talking crap about that field but wouldn't be overly keen to submit to HN questioning. Could well be that maxklein is in a similar position.