Slide toppers (like main awnings) will unroll when wind gets under the leading edge of the rolled up material. The spring loaded tension that rolls it up is all that keeps it in place. The roller might be stationary, but the fabric will unroll. That is what you experienced.
Many slide toppers have an anti- billowing arm mounted on the leading edge of the roller. If wind tries to unroll the fabric, the arm contacts the side of the Rv and stops the roller from turning. If yours has one, make sure it is adjusted correctly. (Search u-tube for installation videos).
There are also locking/latching devices available for your main awing.
I had my main awing try to unroll on me on the highway. I saw the arms bending out away from the Rv in the mirror. Slowed down and it stopped. Found a Dollar General store and bought bungees. Used a bolt from my "junk", box, put it in the hole in the awning roller and bungeed it to the pole. I made a lock for the awning after we got home.
If the topper is okay, keep it!
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Mike and Carla
1998 42' Overland Larado Diesel Pusher
On the road - here and there - Full Timing