I had a similar experience recently..................

Arriving at IAD the immigration official, asked the usual questions about how long and why and then started asking more pointed questions about my travel history.
I am a UK citizen, who owns a vacation home in Florida. I have a pretty unusual work arrangement, where I work for 28 days straight, then I have 28 days completely off. This was my 3rd leave in row travelling to US to spend my leave there rather than UK. I normally travel on my own since my wife will have travelled to FL a couple of days before to open up the house.
(Interestingly, she spends more time there, but same amount of journeys, but she uses TPA or MCO and has never had a problem)
Anyway, the immigration guy then proceeds to tell me that he is considering sending me to secondary investigation for abuse of visa waiver program. He asked me several more questions about my work, before saying that he will allow me entry but that I need to get a visa (B1/B2) for any future trips.
I have been a regular traveller to US for many years and have never overstayed or had anything similar occur.
The actual experience was quite unsettling at the time. We went to the US embassy last week and picked up the recommended visas with no issues, so hopefully my next trip in September will be straightforward.