The Canon AE-1 uses a combo magnet/electromagnet to hold the shutter in the charged position.

This magnet is deactivated by a small electrical charge (electromagnet) which creates a canceling effect on the magnet causing it to let loose the bar which then allows the shutter to fire.

If something is in-between the magnet and the bar it can prevent the magnet from holding the bar causing it to release after you have wound the shutter charging/film advancement lever.

If you are careful you could try accessing via the bottom cover the shutter magnet/electromagnet and with a moistened strip of paper with reagent quality Isopropyl alcohol 85% or better, wipe the surfaces. If the magnet still won't hold you will need a new unit which will require getting to a camera repair shop.

Look online here: Canon