wake
as some have already put it, most likely it was wake turbulence
i was once flying a C-206 and my friend about 8 nm ahead of me in a cherokee 6-300 same level FL055, same track...smooth, cool, morning, cavok
it lasted about 10 secs, i lost almost all aileron control...ailerons became sluggish like the aircraft was stationary on ground
helpless, with my feet on rudder pedals i came out of it without wing drop(s)