I'm guilty of this (as a driver) just because I've had so many bad experiences with cyclists. I work and commute (both by bike & car) in a city that's known of it's cyclists.

Davis, California - Wikipedia, the free encyclopedia

Most of these cyclists are students and in the fall, it's particular terrible. They're new to the area, they're new to cycling and there's a very high chance of ignoring all posted signage and their immediate surroundings.

Just the other day I'm driving 15 MPH (in a 25) with a student cycling about 50 yards up ahead on my right. They seem unstable and distracted on the bike. As I approach I begin to 'creep' and sure enough, out of the blue (no hand signal, no look over the shoulder and no mirror) they make a near 90 degree turn across traffic and flip a u-turn in the middle of the road. The ongoing car in the other lane slammed on their brakes to avoid killing them and I had I been going 25 I'm almost certain that I would have hit them.

I have those encounters with clueless cyclists in town about once a month and the cyclists are all ages and skill levels.

So it's definitely not always an insecure driver. Sometimes it's people that have been on the receiving of insecure cyclists.