Welcome to BYC!

I have crested girls myself. There are two reasons for what you are describing (unless they are just grooming each other, which chickens sometimes do)... The only difference is motive: Either it is a display of dominance or it is restlessness/boredom... Sadly, crests are often targeted no matter the motive since they are big and easy to grab.
Dominant behavior:
When chickens are establishing their dominance, they peck one another (most commonly on the head/hackles). When chickens that are lower in the pecking order get pecked, they freeze, run or challenge. With cresteds, I have noticed that they do the 'freeze' tactic to show their submission... Thus, I will have all my cresteds sitting in a circle and have one peck the other... and the pecked one stays still while the dominant lands a few more pecks for good measure.
Boredom
The other reason could be boredom. When chickens get bored and decide to peck each other, the one being pecked is under the impression that dominance is being displayed... so they either freeze, run or challenge. Since polishes commonly 'freeze', boredom pecking continues without intervention.
Some of my cresteds have white-tipped crest feathers... which are are constantly the subject of interest... and since they always 'freeze', the others just keep trying to pull those white tipped feathers out without intervention. However, since they are free-range, they have few problems and their crests stay full.