I, ah, I don't really understand it myself but he was following Paradox through his time jumps after his card was stolen. He picked up Juudai-kun somewhere along the line and then wound up back in my time where everything that was going to change the future started.

[Yuugi didn't really get the whole thing - it wasn't like he was the best at school after all. Kaiba-kun might have, but the young CEO didn't believe in supernatural stuff, so talking to him about people jumping through time would have ended in Yuugi being unceremoniously thrown out of KaibaCorp.]

Since it took the three of us, I assumed we needed to do it because he messed with our points in time.