Ascend Charlie says:
Why did airspeed have a say in it? Because the vertical fin, canted off to one side, creates thrust to unload the tail rotor - remember how much left pedal you need in the hover, but when you are pulling the same power in forward flight, the pedals are much closer to neutral and might even be slightly right pedal forward.
Just a nit - not all fins are canted (I don't think the R22 is, the R44 & 66 are). You can kind of tell by how an R22 flies at cruise - still needs a fair bit of left pedal compared to a 206.
Also, some (like Enstrom) don't really have much fin at all.
The original question asked about why the left yaw, and I agree that what people have been mentioning are the biggest factors, but nobody mentioned transmission drag, which is typically what's yawing your nose left during the flare of an autorotation. The friction inside the gearbox tends to want to drag the nose, but is typically being countered by torque, fin, and pedal. But during unpowered flight, and especially when you wind the RPM up, the drag is sufficient to be noticed, although not a huge effect (if there's any crosswind it'll typically hide any transmission drag effects).