Why would he have been under surveillance two years ago?
He was not running for POTUS two years ago. Was not considered a serious candidate by most one year ago.
Trump has been talked about as a candidate for 20 years. Even Oprah has asked him about it years ago. But what specifically probably caused this is that he supported Romney in 2012 and made a stink about Obama’s birth certificate.
Trump was considering a run in 2011. That's also when he went Birther.