This is usually due to the environment becoming slightly damper, the paper absorbs the moisture and expands. It hits the rebate of the frame, can’t go any further and so cockles. Cockling also occurs if the paper is thin or poor quality, this is a problem to do with the item to be framed not the framing itself.
So, what is the answer? If the item is on thin paper, then it can be stuck down onto a board. However, although this is probably the most effective solution, because it is irreversible it should only be used with pictures that are replaceable.
If the work is in a mount and cockled, then this is usually a result of the way the picture has been taped to the mount.
If the work has some value then consult a restorer.