I think condescending feedback = insecurity is a bit of a generalization.Maybe sometimes it is true but not all the time - for example Linus' "feedback" on CVS was pretty condescending but I doubt that was because he was insecure
I suppose the insecurity doesn't have to be about domain knowledge, either - it could be about communication skills. Perhaps some people giving condescending feedback don't feel like they are able to communicate in any other way. They experience the desire to contribute, but aren't confident in their tutoring skills and don't want to risk sounding stupid by bungling an explanation.
This is admittedly pure speculation...