Most likely wake turbulence...the aircraft flew through the disturbed air behind another plane (imagine the wake behind a boat, sort of like that but 3-dimensional). Wake consists of two spinning vortices...if you fly into one at a certain angle the vortex can cause the airplane to roll. Not a big a deal on the descent, you just level the wings again. But on short final near the runway it can be catastrophic if a smaller plane gets in the wake of a larger plane...the roll can exceed 90 degrees and there may not be time to recover due to low altitude...for this reason controllers maintain specific separation between planes in the approach environment.

Weather-induced turbulence is usually more of a bump than a roll (like a speedbump) although at low altitudes on a hot day or windy day you can get turbulence induced rolls.

But a roll out of the blue so-to-speak was probably a wake encounter...not considered abnormal or hazardous at higher altitudes, although controllers should not put a small plane into a position to hit wake from a significantly larger one.