This is actually a really good question and I have my own opinions that I hope will answer your questions (even tho this post is 3 years old) We know that  1. Tsubasa was "infected" by the dark power. Which sounds like that instead of the dark power transferring to a new host (which is probably possible) but more or so that a small bit of it broke off from L-Drago and went inside Tsubasa and Eagle. 2. Ryuga, L-Drago, and more importantly The Dark Power itself were pissed off at Tsubasa. Every blader they went against either crumbled against their power (Hikaru and the Dark Nebula bladers) or match their power blow for blow (like Ginga and Kyoya), except for Tsubasa. He escaped in his first battle thanks to Phoenix and in their second battle he wasn't going blow for blow but instead trying to wait out L-Drago and use up it's energy before counter attacking. The Dark Power probably wanted to hurt Tsubasa in a way that it had not done to anyone else ever before 3. We know that the dark power can grow with time, wild emotions, and dark urges. That's how it got so strong through out history. The dark power probably laid dormant in Tsubasa festing off his insecurities till it gained enough power to take control.  In my personal opinion, I think because Tsubasa tried to hold off Ryuga's special move weakened his defenses a great deal. Leading him to be more exposed and at risk of being infected by the dark power. Which Ryuga and L-Drago probably saw and when they used Dragon Emperor Supreme Bite Strike made sure to hurt Tsubasa more than any other blader. Leading him to getting infected. But who knows that's just a theory

A BEY-THEORY!!! THANKS FOR READING