I think it is some mechanism she has been taught to use to try to cope with her eyes acting up. She does other things like when she starts to freeze up she looks down or looks to the side or closes her eyes tightly. I believe others who have experience with Parkinson’s patients have pointed out exercises or things people with Parkinson’s can do to stop when they freeze up for other reactions.
A friend uses Tai Chi movements to unfreeze and it works.