I got pulled over in ORD couple of years ago - was dating someone there at the time and made several visits through the year, usually only 10 days at a time. When they queried why I was visiting, gave the usual "vacation" answer which was followed by how can I afford to visit so often - both financial and time of work - advised my salary and my annual leave (work for UK government and get total of 48 days including holidays plus flexi) which didnt go down well, as I know many US will fall over at that !
Anyway I get cleared and see that the problem was that although you get the stamp when you go into the country - it was appearing I was there every 2-3 months, so they may have thought was over staying nearly the 90 days, flying back to UK then back out again.
Funny thing was I left ORD week after being questioned, then flew back to MIA a week later and was welcomed with open arms!