Some games may include random events. These are usually rare, but not always -- be sure you know the odds!
For some types of units in some types of terrain, there is a chance for it to have an accident that wrecks or eliminates the unit instantly. This depends on both unit type and terrain type. If the accident occurs, the unit is wrecked or vanishes along with all its occupants. Wrecking and vanishing have separate probabilities. Occupants may survive wrecking, if they can escape and survive on the underlying terrain, but if the unit vanishes, they automatically vanish also.
Attrition is "slow death"; it takes away some number of hit points each time it occurs. The rate of attrition depends on unit type and terrain or transport type. Very low attrition rates may only take away one hp once in a while. When a unit reaches 0 hp because of attrition, unit becomes wrecked(?), and any occupants have a chance to escape. You are not notified of hp loss due to attrition.
In a revolt, the unit changes sides spontaneously, perhaps to independence, perhaps to the side of a nearby unit. Occupants will change over if possible, or else escape if possible, or else be killed. Any plans will be cancelled.
Surrender is like revolt, but is always to the side of a nearby unit that is capable of attacking and/or capturing. The capturing unit does not move, however. Occupants of the surrendering unit may either change sides also, escape, or die, depending on their types.