Per Steyer's impeachment ad he alleges
Trump obstructed justice by firing James Comey. Trump admitted the firing had to do with the "Russia thing"
Trump is in direct violation of the Constitution because he's taken money from foreign governments (all his hotels and golf courses aren't free you know...folks pay money to be there and foreign government folks visit all the time)
AND Trump threatened violation of the First Amendment which occurred when he threatened to take away broadcast licenses because those stations don't suck up to him like Fox does.
Steyer also said Trump has brought us to the brink of nuclear war. I think he got that from Senator Bob Corker who said it first.
So reasons ARE provided in the ad that Steyer thinks are impeachable offenses. Whether the courts would agrees is TBD.