Quote Jack Ruston:
...a weird imaging artifact occurs with the crash cymbals. One seems to swing across the stereo image suddenly.
I've heard this too, and I think it is caused by the physical movement of the cymbal. Cymbals tend to have a 'null' in their 'polar pattern' which extends on the plane of the cymbal's rim. If the cymbal is walloped and then tilts and swings on its stand, that 'null' may well point briefly in the direction of one or other of the overheads, resulting in a brief dip in level and a corresponding violent image shift.
Technical Editor, Sound On Sound