The Admiral explains the two main conditions when someone holding a call option or put option may be assigned the stock. He answers when and why will someone exercise an option. The Admiral is a former CBOE Designated Primary Market Maker (DPM).
This an excerpt from the "Trade Options like a DPM Webinar #1 - Covered/Buy Writes" Q&A session: http://hamzeianalytics.com/pow_register.asp