That's not to say there aren't other valid reasons, however.
Special Counsel Robert Mueller is hopefully finding a lot of them during his criminal investigation into Trump's business and other practices. Earlier this week, The New Yorker magazine published an absolutely excellent, in-depth article by Adam Davidson (link here) about a previously little-reported and potentially criminal business deal between Trump and nefarious Russian parties in the Republic of Georgia that potentially involved bank fraud and money laundering.
This is the kind of potentially criminal behavior for which Mueller has assembled the best possible prosecutorial team. Trump's lawyers have tried to argue that investigating Trump's business is beyond a special counsel's law, but the law overwhelmingly is not on their side.
Quite simply, Mueller has all the powers of a federal prosecutor. And a federal prosecutor has the legal authority to investigate any and every suspected crime, no matter what that crime might be.
The Trump legal team's claim is as ludicrous as saying a federal agent couldn't arrest someone for murder, when evidence was found, if the agent started out investigating a bank robbery.
I urge you to read The New Yorker piece and share it with friends and associates. This is the kind of criminality that will bring Trump down.