What sources of damage are not caused by players?
Whenever a player plays a card  that instantly deals damage to one of more players, that damage is considered caused by a player. This includes all possible brown cards with a 'bang' symbol.

If the damage is delayed (as it is with dynamite), it is not caused by any player. This is probably partially because it is impractical to keep track of who played it (especially when there are multiple dynamites going around).

That being said, dynamite is the only card in the base game that has this property. Off the top of my head, the card 'Rattlesnake' in the Valley of Shadows expansion is the only other card I know of that has similar behavior.