First let's distinguish On-Hit/On Monster Hit versus On-Hit Cast Spell
On-Hit/On Monster Hit are hardcoded. They occur only if the target sustains physical damage, and sometimes occur only a percentage of these hits (typically 25%). On Hit Wounding occurs every hit that deals physical damage. It is resisted by a fortitude save (d20 + fort save vs. DC).
On-Hit Cast Spell is scripted. It occurs every hit regardless of damage dealt. Only one On-Hit Cast Spell can occur at a time.