Why do guys unmatch girls on Tinder?

There is no absolute answer to this question. I cannot speak on beahlf of all guys. I can only write the reasons I have unmatched girls on Tinder.

There was no genuine interest

Many times we are not selective at all. We just swipe right to all girls until we cannot anymore and then wait to see which girls matched us.

Of all our matches we choose the ones we most like. Every match demands us time, effort and energy.

He didn't like the conversation

I have unmatched girls, who made me feel uncomfortable while talking with them.

Sometimes I just felt they were too demanding, or they have extremely high expectations.

He wanted to unmatch you before you do it

I have unmatched girls, who made me feel they would unmatch me. For example:
She: Do you have a girlfriend?
Me: No, but my ex girlfriend and me still behaving like a couple.
She: Then you have a girlfriend!
Me: More or less, no and kinda yes.
She: I am so tired of talking to guys who have girlfriend, that's unhealthy!
Me: I see, though you can make friends and transmit good vibe :)
She: You sound like my father giving me lessons of personal superation!
Then I just unmatched her, because I felt she would unmatch me.

Trying to see your reaction

I have unmatched girls who I liked, even after having some kind of good conversation. I just felt like:
Once one of us deletes and creates the account again, we will match again.
I clearly remember a girl, I sent her a superlike, she said yes, then we matched. I would say, that she was nervous. At least she seemed to be.

When the conversation was a little stucked, I just unmatched her. Then I deleted and created my account again.

I was waiting for her. I knew her distance and said no to all girls, so that my list of girls nearby were empty. I was looking only for her. She probably felt frustrated because I unmatched her.

When I found her again, I sent her a superlike again. She said no. After that, all times that I found her again, I said no to her.

It took me a lot of days to understand why did I lose the interest on her that way.

I matched recently a girl, who I unmatched her in the past already. I made her exactly the same questions and she replied with the same answers. Then I asked her:
Me: Did we have a match in the past?
She: I don't know XD
After that moment I started to like her.

After trying to understand myself, I concluded, that I unmatched girls I liked, in order to know their response.
I don't care too much about you!
The above attitude looks sexier, with more self-confidence and more challenging, than one taking the unmatch seriously.