Or your manager arbitrarily decides they don't like you (particularly in startup-like environments with limited process). It shouldn't happen, but it does.
My friend who has wider experiences changing jobs than me also said the same thing as your comment and its parent. Workplace politics is a real thing.
Not only is it a real thing, at some workplaces it's the main event.
Depending on where you live 3-6 month probation periods can be completely standard. If you get the job, you are accepted there. The probation period is just a window in which you and the company can separate with less friction. In most places it's a formality that you shouldn't worry about unless you're extremely underperforming or doing something egregious at work.