I sometimes have this issue - for me it comes from searching for the ball and trying to play it too far in front of my body. I'm effectively slightly early on it and so if playing into the offside, the bat is past the point at which the middle would connect, and it comes off the inside half. Other times with straighter balls I get to the end of my 'reach' before the ball arrives and the bat face opens as the bottom hand still has distance to travel (if that makes sense). I tried fiddling with grip etc but for me it was fixing the symptom not the cause.
Particularly happens early season when I've not got my timing back - this week I am doing a few feed buddy sessions with the 'no feet' drill to try and re-teach myself to wait for it.