Although its easy to assume that Wheelies are to blame for such a thing, don't underestimate the power of vibrations under load.
That is; the engine is stronger than the frame. If the frame is clamped to the engine and under stress, in due time it can crack. The possible preventive measure is to loosen the engine mounting points (all of them) and allow the engine to settle into a "NO LOAD/NO PRESSURE" position and re-tighten the bolts.
That being said, I have nothing to loose by loosening the swing arm, engine mounts, and rear brace to the tail section to make certain the frame is not being stressed. If your bike has ever been down, this would be a good preventive measure.
IF you want to know if it moves when the bolts are loosened; place some tape over the jointed sections and see if it wrinkles. i.e. between frame and engine at the mounting points and the rear brace.
That is; the engine is stronger than the frame. If the frame is clamped to the engine and under stress, in due time it can crack. The possible preventive measure is to loosen the engine mounting points (all of them) and allow the engine to settle into a "NO LOAD/NO PRESSURE" position and re-tighten the bolts.
That being said, I have nothing to loose by loosening the swing arm, engine mounts, and rear brace to the tail section to make certain the frame is not being stressed. If your bike has ever been down, this would be a good preventive measure.
IF you want to know if it moves when the bolts are loosened; place some tape over the jointed sections and see if it wrinkles. i.e. between frame and engine at the mounting points and the rear brace.