From what I gather a repeat visit soon after the first one can be a red flag and depending on the situation could be a reason for denial. Not saying that was the case here but could have played a part along with other factors.
A friend of a friend of mine visited the US more than once without much time in-between- he just had time on his hands and liked visiting and exploring. After the third or forth time he was admitted but with a warning that he may get denied admission on a future visit to soon.
I was at LAX some years ago and heard an officer questioning a traveler for a while. He was eventually let in after a lengthy conversation and I then overheard her telling another officer that the guy had visited the US 12 times in the last year and she was getting suspicious.