Cupping is a humidity response from the leather. Edges dry quicker than the centre. I keep my strops in the bathroom on the hook but hang my "in-use"shaving towel over the hook also so it covers the strops. This I found eleviates the problem arrising.
To correct the cupping long term you need to lay flat on a surface, cover with a dish cloth. Rotate the strop every 6-8 hours or so.
The quick way is as pointed out, roll the edges. I would recomend you regular rub the strop with the palm of your hand to oil the strop and at the same time make your hand press more on the edges so in effect cupping the strop in the reverse direction. kill 2 birds with one stone this way. It is a good habit to get into anyway, rubbing your palm over the strop till your hand gets warm prior to stropping. This oils the srop and sets the draw straight off.

PuFF