From blogsouthwest.com:
"Mr. Smith originally purchased two Southwest seats on a flight from Oakland to Burbank – as he’s been known to do when traveling on Southwest. He decided to change his plans and board an earlier flight to Burbank, which technically means flying standby. As you may know, airlines are not able to clear standby passengers until all Customers are boarded. When the time came to board Mr. Smith, we had only a single seat available for him to occupy. Our pilots are responsible for the Safety and comfort of all Customers on the aircraft and therefore, made the determination that Mr. Smith needed more than one seat to complete his flight. Our Employees explained why the decision was made, accommodated Mr. Smith on a later flight, and issued him a $100 Southwest travel voucher for his inconvenience.
You've read about these situations before. Southwest instituted our Customer of Size policy more than 25 years ago. The policy requires passengers that can not fit safely and comfortably in one seat to purchase an additional seat while traveling. This policy is not unique to Southwest Airlines and it is not a revenue generator. Most, if not all, carriers have similar policies, but unique to Southwest is the refunding of the second seat purchased (if the flight does not oversell) which is greater than any revenue made (full policy can be found here). The spirit of this policy is based solely on Customer comfort and Safety. As a Company committed to serving our Customers in Safety and comfort, we feel the definitive boundary between seats is the armrest. If a Customer cannot comfortably lower the armrest and infringes on a portion of another seat, a Customer seated adjacent would be very uncomfortable and a timely exit from the aircraft in the event of an emergency might be compromised if we allow a cramped, restricted seating arrangement."
It looks like the incident was cause by the circumstances of his ticket. They're saying that it wouldn't have happened if he hadn't elected to fly standby.