From Left to Right

I'm sure it's happened to many people. They consider themselves mostly liberal in their leanings, but maybe have a contrarian streak. There are some ways of stating the liberal positions that are off-putting. Generally, these will be minor annoyances at first. These people consider themselves open minded, so they welcome the more intelligent commentary from the other side, and are willing to consider other points of view. We know the saying about a liberal not being able to take his own side in an argument.

At some point, the intelligent position on the other side starts to seem appealing, in some ways.
The person starts a lot of sentences with, "I'm liberal, but..." For some people, there will a tipping point, where the old liberal positions don't seem as appealing. They will be former liberals.

Because of the structure of political opinion, it is harder to stay in the middle. People with  liberal views will line up with every other liberal in a predictable way,  and the same goes for the conservative side. There is a certain coherence, or seeming coherence, in the views of all the issues. This is because the opinions are not really held as beliefs, in many cases, but as markers of identity in a political faction. That is why someone can change factions and then change all their beliefs at once.

This hasn't happened to me. I do get annoyed, but in part it's just my natural crankiness.