Are joint hypermobility and tendonitis related?

Sort of. There is some evidence that those with hypermobilty are at increased risk of sprains and strains, although there also exists some evidence to the contrary. Pathophysiologically speaking, I am not aware of any studies implicating changes in the structure of tendons among individuals with hypermobility.
Joint hypermobility. Yes hypermobile joins can cause tendinitis, bursitis, frequent sprains, early onset osteoarthritis, subluxation and dislocations more common in shoulders, knee pain, herniated disc, clicking joins, TMJ syndrome, carpal tunnel symdrome andgrowing pain in children. All these are caused by instability in various joints.