Now that Donald Trump is the president-elect, Democrats and liberals are trying to figure out if and how he might get impeached. To be clear, Trump hasn't taken office yet, and so he hasn't had any chance to commit an impeachable offense yet. A Trump impeachment could happen in a specific scenario, but not many others. It would require a perfect storm of factors to collide, with multiple disparate pieces falling into exactly the right places at exactly the right times.
It's impossible to say what the actual reason for Trump's hypothetical impeachment would be, simply because there are a theoretically infinite number of things a president can do that qualify for impeachment. The Constitution says that a president and vice-president "shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors," and that covers a whole lot of ground.
But hey, it never hurts to speculate. So maybe Trump becomes implicated in a pay-for-play scheme or some other manner of influence peddling while in the White House. Or maybe he enacts draconian "national security" policies that infringe on Americans' Constitutional rights. Politico envisioned a situation wherein a President Trump illegally detains Muslim Americans in internment camps, for instance. One could imagine a scenario wherein something is revealed about Trump's ties to Russia that constituted treason, though this seems extraordinarily unlikely. Perhaps he makes good on his support for killing the family members of suspected terrorists and is tried for war crimes.
Once again, all of this is pure speculation. There are an unlimited number of reasons Trump, or any president, could theoretically face impeachment charges. But let's suppose he were to do something that, in the eyes of legal and Constitutional experts, was grounds for impeachment. That wouldn't be enough. The House of Representatives is responsible for bringing formal impeachment charges, and Republicans control the House. In order for Trump to get impeached, Republican representatives — or a majority of them, at least — would need to feel massive political pressure from their own constituents. And they wouldn't feel this pressure unless Trump himself was massively, massively unpopular with all factions of the American public — including the people who supported him during the campaign. He'd have to be hemorrhaging popular support at an astonishing rate, with polls showing that a large majority of Americans, including his own supporters, had turned against him entirely.
If this were to happen, the House could conceivably file impeachment charges. But that wouldn't get Trump out of office. Remember, "impeachment" refers only to the formal charge of wrongdoing; it's the Senate that actually tries those charges, and decides whether an impeached president gets removed from office. This could actually be an easier lift than in the House. Unlike representatives, senators only face reelection every six years, making them more immune to the whims of their constituents. If you accept as a premise that most Senate Republicans are reluctant Trump supporters at best, and much rather have Mike Pence as their commander in chief, that would give them them all the incentive they need to approve impeachment charges.