I'm pretty sure that while no one knows the cause definitively, many people agreed that the far more likely explanation for the bit change was a hardware fault (memory error, bad cartridge connection or something similar) or other, more powerful sources of interference. The player that recorded the upwarp had stated that they often needed to tilt the cartridge to get the game to run, showing that the connection had already degraded. The odds of it being caused by a cosmic ray single-event upset seem to be vanishingly low, especially since similar (but not identical) errors have already been recorded on the N64.