Causes

A unit must test for Panic immediately if it loses 25% or more of the models it started the phase with -- most often from shooting or enemy spells, but any source of casualties counts (miscasts, misfires, failed Dangerous Terrain tests, and so on), used as a catch-all rather than listing every specific trigger.

A charging unit that's Stood & Shot at and takes 25% or more casualties this way must also test immediately -- even if that leaves it panicking while technically still out of the shooters' range. Treat this as the charger having been shot at the moment it entered range, panicked, and fled back out of range again, rather than working out the individual moves involved.

If a unit that started the phase at Unit Strength 10 or more is destroyed for any reason, every friendly unit within 6" must immediately test for Panic -- leave the destroyed unit in place until the tests are resolved, so there's a fixed point to measure from.

If a unit that started the phase at Unit Strength 10 or more breaks from close combat, every friendly unit within 6" must immediately test for Panic -- measured from its position before it makes its flee move.

A unit must immediately test for Panic if fleeing friends with Unit Strength 10 or more move through it -- resolve the fleeing unit's move first, then take the test. This is Panic at its most destructive: one unit fleeing through another can chain into a third, and so on, until a whole battleline unravels.