The basics: Moving away from a threatened square provokes an Opportunity Action.
- Certain forms of voluntary movement (shifts, teleports) specifically do not provoke OAs.
- Being pushed, pulled, or slid by someone else's power (forced movement) also do not provoke OAs.
- What if the movement is involuntary, but not forced by another PC or NPC? For instance, falling due to failing (or skipping) an athletics/acrobatics check, being swept away by the current, or other terrain hazard?