My guess on what happened with Leon Wilkeson is this. He was already playing bass in the upright (Bill Wyman) position anyway even before the plane crash. But in the crash his arm, of course, got very damaged and the doctors considered amputating it. After the injury, he was never able to fully extend his left arm--making it impossible to play a bass any other way except vertically. Most likely, though, he had to play at an even steeper angle than before.
I can identify with this a little. Because of whiplash injuries playing pickstyle puts some pressure on my neck. I find myself with no choice but to sometimes "try" to play fingerstyle, even though that has never been my preference. But the change from pickstyle to fingerstyle can make a big difference on my neck.
