It is always due to wear, but the more usual gear is 2nd. Additionally, extra damage is done by the act of "jumping" so you do NOT
want to ride again until some type action is taken.
On the 650/750 twin you can try a short cut, quickie fix of replacing the 5 coil springs
. This since gearboxes are prone to collecting water that attacks the coil springs, and general spring collapsing arising from 30 years of being under compression.
The camplate plunger spring is inside the dome-shaped nut on the bottom of the gearbox. (Next to #3, below.)
Then there are 4 shifter springs inside the outer cover.
This is a very inexpensive repair, and one you'll end up doing anyway. Now the modern springs will not be exact copies of the ones you take out. This is due to wear (as mentioned) and newer sping manufacturing methods. Not to worry, just replace them.