For luck and morale it's around 10% per point in morale luck.
For creature abilities it's like this: The probability of triggering depends on HP of damager and target.
If DHP ≥ THP, then
Probability = 25% + 3% * (DHP/THP);
else
Probability = 25% - 3% * (THP/DHP)
where:
» DHP is the total remaining HP of the damaging stack;
» THP is the total remaining HP of the target stack;
» Trigger probability lays within the interval of [5%;75%]
There are also some talents which can improve possibility of troop triggered events. I can't remember off-hand the talent names (or which factions can select them) but most likely someone will post this info here.
