So why does he decide to be a jerk at the wedding?

He didn't think he was good enough for her so he acted like a complete jerk to push her away.

On the day of the wedding, Tank overhears Alexis say that she's falling for him. Tank feels guilty for being disloyal to Jason Biggs's character, so he sabotages the wedding with the intention of sabitaging his relationship with Alexis.

The scene was a little confusing at first, but it was over the top with all the puking, etc. I almost turned the TV off...

I think it was meant to be his ultimate "Tanking"

You found out later that he was on the phone to Dusty and therefore it makes me think that it was all to push her and Dusty back together

Do guys like "the thing"?
They like it better than no thing.

While I found the scenes over the top but somewhat funny I felt they were not needed for the character. He could simply have broke it off with her in private saying he wanted to move on and didn't have feelings for her.

I thought it might be so her sister didn't marry the guy but that only worked when Jason Biggs' character comes in and confesses everything.

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