I actually have no idea what exactly started it because I have always had an interest in anthropomorphic animals and have been writing/drawing/making characters since a very, very young age.
My first drawings were all of cats, though not necessarily anthropomorphic, and I was like, two years old. (At least I think I was since that's what my mom said, and it was before I even started talking . . .) I think I was somewhere around the age of four when I "made" my first character (though he was based on a stuffed animal) and, especially after I got more stuffed animals to add to this cast of characters, I started writing these "stories" about them, some of which I still have today. (Though most of the stories were incomprehensible drawings put together with no text, and only I can understand what is actually happening in them). These characters were this sort of mix of anthro and "feral", because they often stood up like anthros, and they acted anthropomorphic, but they also at times walked on all fours and acted more like normal animals. Watching a lot of Disney and other animal cartoons greatly influenced my art/stories as well.
2007 was the beginning of my dog phase, where I mainly started writing stories about dogs (which were still anthropomorphic, but lived in a world similar to ours and looked like regular dogs with owners etc.) and I even made a "fursona" in 2010 (though at the time I still didn't actually know about fursonas or furries) but this started to change in 2011, especially by the end of 2011, when I became obsessed with Sonic and my main focus was this weird fancomic (which started out as a joke). Unfortunately for a while, all my art looked like bad Sonic art and rip-offs, I even tried reviving my original cast of characters but they looked like Sonic. I rewrote that fanfic like three more times until finally I decided to stop drawing Sonic and turn my art back into original stuff, this took a while but I have finally regained my art style.