Could be the individual spirals of the hairspring are stuck together, which reduces its moment of inertia & causes the movement to run fast.
Magnetism, oil/grime or an impact can all cause this. As chris5264 said, if you remove the caseback, you may be able to see what the problem is. The spring should be more-or-less centered around the balance wheel when stopped & you should see evenly spaced spirals. Check, too, for spots where the spring may be coming into contact with adjacent wheels (the center/2nd wheel is a prime target if the case is impacted).
As a reference, this is what a hairspring may look like when it is sticking or off-center. Compare the space between the spirals at 4 o'clock with those at 10 o'clock