I was talking about this with my wife, and she said Trump appears to be using "playground tactics". I agreed, and asked her if she knew why he was doing that? The thought for a few seconds, and said "because it keeps working".
Trump takes a low-brow adolescent-style jab at the dems, and they fall for it and make themselves look even stoopider EVERY TIME!
Trump's troll level: Master of the Masters.