Definitely 1440 RPM motor is designed for 50Hz. Capacitor and motor inductance makes resonance circuit and if motor is runing on 60Hz, it is "closer" to resonance. Voltage on "run capacitor" is usually higher than line voltage (commonly known effect with resonance circuit) and closer to resonance can be twice or triple higher than supply voltage. There I see the main reason for damaging of capacitor.
Proper determination of capacitor needs engineering calculation. Ask manufacturer.
Andre
