According to Square D pubs and UL listing, SWD rated breakers can only switch fluoresent lighting loads. The only other thing I can think of is the ambient temperture in the panel. Breakers stamped with "40 degeees C" indicate that it is rated for continious duty. If it is not stamped "40 degrees C" then the circuit needs derating. Breakers espically lighting panels can get really warm. Even if the air outside the pane may be normal, inside a closed metal box, it can get really warm. If you are switching HID lighting, then the breaker must be HID rated.
I have flipped SWD breakers that were old yet outside of the linkage having a worn feeling to them, there were nothing wrong with them.
