Noted psychologist Desmond Morris discovered that the telling of a lie caused a tingling sensation in the sensitive neck and facial tissues and scratching or rubbing was required to satisfy this tingling.
This seems to be a reasonable solution as to why people may pull their collar when they lie and suspect that they have been caught out lying.
It's almost like when they lie and think you know that they are lying it causes a bead of sweat around their collar and pulling the collar let's the air in to cool the neck down.
If you notice someone using this gesture, why not ask them to repeat what they have just said! See if they squirm.