Why do guys come one strong then back off?

Know this guy for two weeks and saw him 4 times the first week. Then he found out I saw someone before we met and the guy told him we were still seeing each other. I don’t even know where I stand with it this guy. He could be seeing someone else. The week after I only saw him once and he’s not messaging me as much. I ask what does he want from me and he just kinda smiled and looked away and said I don’t know. What’s going on with him?
Why do guys come one strong then back off?
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