Why would someone block you for congratulating them?

I take part in local tennis competitions and so does he. We are casually familiar with eachother because of that and did mutually follow eachother on Instagram, with normal interactions such as likes and random comments.

I follow one of the local sports clubs because I take part in their women's tournaments and recently they posted a pic of him because he won the tournament he was playing in. I sent a message saying "Congratulations on winning, that's awesome! Hope you smash the next one too!" and I got blocked. I don't understand why, what did I say that was wrong?

Why would someone block you for congratulating them?
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