I can't comment on your particular case, as I don't know you from Adam.....but.....
The most common cause of this is the American phenomenon known as "The Bench Press Syndrome"...lol....it is the obsession and fixation that the average American weight trainee has with the bench press.
Most people focus on flat benching at the expense of all else....and even people who think they have a 'well-rounded routine' do not put nearly the time and effort into back and leg work as they do benching. What causes the shoulders to round in most cases is the lack of proportionate back strength. Your 90 degree STRICT row should be equal to your flat bench, or at least respectably close.
I would just recommend more back work, and building up your OHP can never hurt either, as this is often totally neglected in the pursuit of a bigger bench by most people.
Again, I don't know if this is the cause for you.....but more times than not it is an issue for people. But, to address the issue, don't pussy foot around with rear delt raises and other isolation stuff. I would row at 90 degrees and keep the motion strict and build it up over time. Chins are good, as are deadlifts, RDL's, jump shrugs and olympic lifts.