I got my yoga butt from doing a cartwheel without warming up first. It took me 20 years to heal it because it took me that long to get serious about it. It was just a bothersome ache at my butt bone that was worse with sitting. Lots of people get yoga butt from doing hamstring stretches (forward bends) without building strength or warming up first. You'd think that the muscle would be more fragile than the tendon but there's a weak spot where the tendon attaches to the bone--this is usually where the injury is. Once it get injured, people tend to keep re-injuring it. Especially yogis who are ever so slightly prideful about their ability to bend.
Tendons get strong from being tensioned, but you have to start gradually. Work out your hamstrings to make your hamstring attachment tendons stronger. This is part of the treatment for yoga butt. The thing is yoga doesn't have a lot of poses that strengthen the hamstrings, but it does have a lot of poses that require you to be able to touch your toes, or fake it. And faking it teaches you all kinds of bad habits that leave you imbalanced, strong in places and weak in others, and prone to injury.
Lots of thoughts about this but I must go.