Usual causes include a round failing to seat properly into the breech, a spent casing getting caught upon ejection (a condition known as 'stovepiping'), poor-quality ammunition (insufficient pressure to cycle the weapon) or poor handling while shooting (not enough energy from the firing is absorbed by the hands/arms to let the slide or bolt move far back enough to cycle, known as 'limp wristing'). It's well-known that even the best guns still jam every now and then after repeated firing.

That said, professionals who use weapons say there are only two classes of weapon users: those who have had a weapon accidentally discharge, and those who eventually will have a weapon accidentally discharge this is why basic Gun Safety says to treat any loaded firearm as if this actually is true, just in case you're dealing with a weapon that's damaged or just poorly-designed enough that it is. Never mind that practically all weapons designed after 1968 include a special mechanism to stop the hammer from falling unless the trigger is properly pulled, and that gunmakers had been adding them for a long time before that. It doesn't matter if it's a cheap Saturday Night Special or a professional quality, $1200 SIG-Sauer, count on this one. Bonus point if the bullet hits a Mauve Shirt or other expendable mook.

Any jarring or dropping of a cocked, chambered gun will discharge it.

Evidently, Hollywood doesn't trust the 'big brands' when it comes to guns, as there are two things you can usually expect to see with firearms depicted in fiction:

Why do guns jam