Could be a variety of reasons or combination of them. It's possible a failing capacitor is at least partially responsible- or poor quality contact alloy?
You just got your motor diploma, wear it proudly
The contact points are kind-of small but they are solid copper. If the issue continues then I'll find a better set of contacts (they're cheap) and I'll check the cap. I might do both anyway if I get time. It appears to be a well built motor but it is a Chinese manufacture. After the start cap releases there is about 8 amps through the start winding, which seems to be about right running at 120v. Not much of a jump in start up amps that I can see with my equipment for that circuit. Running amps is right where the label says it should be.
Tony