It's not the why, but it was part of it. Ra's wanted Dad dead, and Tommy Merlyn returned from the dead? That was a target on my back and the back of everyone I knew. Do you remember how easily I had my ass handed to me right after you came home and you had to save us both? Do you remember how ineffective I was against my "kidnapper" in Hong Kong a few years before that? I wouldn't have lasted three seconds in a fight with one of the League on the day I finally had it together enough to want to go home to you. Show up, just to die in front of you again? I couldn't do it then, buddy.
When you came to get Thea, I was a lot more prepared for that kind of fight. I was going to tell you everything. But it wasn't safe enough, I wasn't going to lose you and Thea, and, yeah, the more you hide and lie, the harder it is to get out from under it. So Dad and I followed you and Thea back, and I started looking for the right way to tell all of you without getting us all killed.