Ukrainians who were trying to "take him down" during the '16 election. So being the vindictive asshole that he is, he decided to hold this against the same country that was emerging from corruption. As an extra added bonus he also extracts a little more blood from this turnip by insisting on a bogus investigations into a political rival in order to get himself re-elected.

So, this is all about a perceived slight to his vanity, placing the national security of the US and an important ally at risk. This is but one danger of having such a small minded man in the WH.