His wife was probably already flagged on the SEC system as having access to Amazon financials. It seems fairly likely the trades were marked as potentially suspicious and once the volume hit some threshold they were escalated to a human investigator.
My wife has a series 7 security trading license. The SEC requires our personal trading accounts be flagged with this information along with information regarding my employer. Any trades I make in my employer's stock take additional processing time (along with standard blackout periods etc). I would assume people with restricted access to corporate financial statements are flagged and tracked similarly by the SEC.