Lots of people have an axe to grind with motorcyclists in general due to tolerating others with loud pipes, stand up wheelies on the highway, etc. Doing something as innocent as passing them might set some unstable folks off and they'll see you as a target out there.
Then again, others are just plain jerks who don't realize they're messing with someone's life. I was on a deserted stretch of Texas highway at about 7 a.m. on a Sunday morning. No traffic for miles except for a semi truck in the right lane ahead of me. I thought nothing of it as I prepared to pass. As I got about even with his rear bumper he swerved into the left lane, causing me to stand on the front brakes and lift the rear wheel of my VFR at 80 mph. He got a good chuckle out of it. Would have like to pull him out of the cab by his greasy mullet, but what can you do on the freeway? I just got the hell away. Living around a large city, you come to expect most of the population to be drunk, high, and packing weapons at any given time.
If you bumped into this guy's cart in the grocery store I don't think he'd go nuts like that. Being in a larger vehicle triggers some kind of power trip in some people. Another reason I don't ride much in traffic.