I followed this race very closely and bet on it at PredictIt.org. So, I had to focus on it very closely. 

The players on PI were shook up when president Trump came back and decided to actively campaign for Strange AFTER the initial primary where a large field of candidates yielded no one candidate who recieved over 50%, thus a run off.

Trump endorsed Strange PRIOR to the run off. Strange was McConnells boy, and the third  strongest candidate was a current congressman from the Huntsville area in the north, Mo Brooks, who happened to be a virulent Never Trumper, who had personally attacked president Trump. 

So, Trump's involvement in the race may have been prompted by McConnel, along with Trump's desire to see that Mo Brooks did not emerge. Strange's attack ads focused on Brooks in the initial primary, and it was not at all assured that Strange would comein second. 

Trump was being quite remote after the initial primary, and it was reported that he was reluctant to actively ........