Trump has a lot of radical ideas that even many Republicans vehemently oppose so it makes sense that the Democratic party would take over the Senate as one way to ensure that Trump cannot pass the most ridiculous laws that he comes up with. It also make sense because for many years now it seems that when one party controls the Presidency, another controls the Senate.
The Donald Trump campaign has energized the Democrats more so than Republicans. Trump's candidacy has been contentious and controversial. Many Republicans are at odds and it has weakened the party overall. This gives Democrats and excellent opportunity to take control of the Senate in upcoming elections and possibly the presidency.
If the President does well in an election, the House and the Senate for the party usually do well, too. When George W. Bush was very unpopular, his mid-term election went terribly, and Republicans lost the House. Trump is losing in the polls and he is unlikely to win the election. With that, the Senate will fall into Democratic hands.
The Senate will change hands in the fall election, largely due to Donald Trump's xenophobic campaign. Every day, we read about more Republican voters dropping their support for Donald Trump. In November, these "never Trump" voters will either not vote at all or vote Democratic. With 34 Republican seats up for re-election and a Republican Senate majority of only up by 8 (the 2 Independents vote Democratic), Democrats only have to pick up 9 of those 34 seats to change the balance of power.