"why did my cat scratch me? "

a possibility is that they heard/saw/smelled something that startled them (even if you didn't hear/see/smell anything, cats are very sensitive).
also, cats see us as big hairless kittens, so sometimes they will attack out of nowhere because they are trying to teach you to defend yourself.
this is also why that "*cat reveals belly* *human pets belly* *cats scratches*" trope is so popular. the exposure of the belly (on a cat) is a sign of trust, not an invitation to pet. so sometimes they are just revealing their belly to show trust and not asking for rubs.
a lot of people have noticed that cats don't particularly like belly rubs. so it could have also been that they just didn't like where you were touching.
sometimes, it's best you don't approach a cat. this is usually when it's ears are back, the whole tail is swishing back and forth, eyes are dilated, and it just generally looks tense. when a cat appears like this, it's either hunting, anxious, or over-stimulated.

why else might a cat might scratch their long-time owner, other than the reasons listed? (and no, "it doesn't like you" is not an answer. that is explained in the last paragraph.)