What kind of tape is it? Poor winding and improper storage can do it.
Best thing to do is a library wind of the tape and store it like that for a while. You can either use a spooling feature, or if your machine doesn't have one just play the tape from start to end so the wind is tight and smooth.
Badly worn heads can cause edges to curl in as well. In that case you'll need to wind the tape on a different machine.