Squat help - grip loosens during set

[Daniel's Jack]

Dear coach,

During heavy squats I encounter a problem with the grip of my left hand. Usually at the third or fourth set my grip gets loose and my hand moves to the left (sometimes as far as 10 cm at the end of the fifth rep). As you can imagine this results in a weird shift of balance on my back, I lose my shoulder tightness and it makes squatting harder although I usually complete the sets, but it’s just annoying and I think it might keep me from squatting higher weights (3x5x275 lbs atm).
Is there some kind of common mistake that could lead to this? Or am I just not gripping the bar tight enough? Can it be the initial placement of the hand on the bar? I thought it might be that I’m gripping to narrow and this leads to a force on my hand pointing outward but I have no idea if those two are related.
Any advice is very much appreciated because I think I could do a lot better without this flaw. I am using a thumbless grip and liquid chalk and otherwise my squat is fine.

Greetings,
Daniel

[Mark Rippetoe]

This is the kind of thing that happens when you start off with a bar position that is too low, below the correct position on top of the posterior delt/base of the trap. Move it up 1cm and see if this corrects it.