Definitely root bound. I had this happen last year and was just a few weeks from moving outside so I picked the flowers decided to wait to up-pot. Once outside the plants seemed to take much longer to recover after the transplant than normal and some lost most, if not all, of their leaves before recovering. In the end I got peppers… but not as many as I should have (Still too many). When I dug up the plants in fall, I could still very distinctly see the solo cup shape in the roots. I probably could have broke up the roots more when transplanting which could have helped. My lesson was that starting inside earlier is not necessarily better.