I can only think it's a neighbour who may have to make extra effort getting into their driveway or can't park outside their property when you're there?
Opposite us lives a guy who parks his van on the road directly opposite our driveway, it didn't really bother us much at first but several months down the line you kind of get a little fed up what's more our next door neighbour's mother inlaw has taken to parking up the verge now to avoid it even though they can park 50 yards down the road and is churning up the grass and it's looking awful. It also means we can't park outside our own house when popping in and out so have to go on our driveway every time we come home. I've no right to complain to anyone the guy's vehicle is fully taxed, I wouldn't leave letters on his van even though he's within ten feet of a bend too but hopefully reading this people could understand more.
Are there any issues you can see that might be similar, is there enough access for a say a dust cart to get down the street without causing any problems to the verges etc?? It might not look an issue when you leave your car there in the morning and drive away at night but something may happen during the day that is clearly annoying someone.
People get a little territorial to a degree and this person clearly has somekind of issue. I'd get your friend to park where you do from time to time and park on their driveway and see what gives.