What are typically some causes of this type of thing?
New rider, operator error. High-RPM clutch slippage, instead of gently pulling away from a stop with the revs just above idle to minimize slippage.
Clutch cable and/or release mechanism not adjusted properly, either not disengaging fully, or not allowing it to engage fully. An experienced rider would detect symptoms prior to complete failure, such as clutch engagement at an abnormal position in the lever travel, or slippage under load, or "creeping" when in gear with the clutch pulled in. This can also cause improper gear shifting, e.g. the previously reported inability to shift past 3rd gear.
Beware when adjusting clutch mechanisms that some of them have two adjustments ... one for the slack in the cable, and another for the free play at the actual mechanism that disengages the clutch. (Yamaha FZR400 and 600 are like this) You can adjust the slack in the cable to all eternity, if the actual clutch disengagement mechanism doesn't allow the clutch to engage fully then it will slip under load.
For "slipper" clutches (which the Ninja 300 has!) - Incorrect assembly. Many of them are quite sensitive to the "stack height" - the total thickness of all the clutch plates together - and require select-fitting steel plates of differing thickness in order to work properly.
Incorrect assembly in general. On my Yamaha FZR the pressure plate will physically go onto the inner clutch hub and bolt up 5 different ways, but only one of them will work ... the one that has the alignment marks lined up!
General abuse and neglect. Use of engine oil with inappropriate specifications. Stupid oil additives (snake oil). Abusive riding habits, whether the owner realized it or not. Excessive clutch slippage under load. Substandard aftermarket parts. This list could probably go on forever.