(off) on a sidetrack
1. Addressing a topic or topics not relevant to the main discussion. I tried to address the customer's problem, but she kept going off on a sidetrack and I couldn't understand what her true complaint was. In the middle of our conversation about my finances, my advisor went on a sidetrack about current events.
2. Branching off into some entirely different field, profession, or pursuit. I actually studied and trained as a pianist before going off on a side track into metallurgy. Our company was originally focused on developing new, original games for modern hardware, but about 10 years ago we went on a sidetrack and just kind of became a full-time porting company for other studios.