go off on a tangent idiom (UK also go/fly off at a tangent)
突然离题;突然离开原来的思路
to suddenly start talking or thinking about a completely new subject
It's hard to get a firm decision out of him - he's always going off on a tangent. 很难从他那里得到一个明确的答复——他老是突然离题扯别的去了。