As most PTs will tell you, it's a very rewarding field and has a very high job satisfaction rate. Thus, most don't burn out as there is so much to learn as the field continues to evolve.
I would guess that those who do manage to burn out either have become stagnant and don't want to evolve with the field or are stuck in a location without much diversity in patient population (not enough stimulation for the amount of work put in). I bet some even burn out by working extra hours to pay off loans in which they don't see enough return (eg. have little time to enjoy life outside of work). Some parts of the field can be very mentally draining, especially in long term care facilities where patients can pass away at any second. Oh, and finally, I'm sure that many have unrealistic demands from bosses, companies, or even from themselves and just can't seem to meet expectations (patients per day quotas, etc). That said, I believe these could be reasons for anyone to burn out in any career.