Nervous Great Spotted Woodpeckers

Can anyone suggest why it is that virtually every regular garden bird species recognises that if I'm walking towards the bird feeders carrying peanuts and seed they will gather expectantly, knowing that food is on the way and that I pose no threat, yet, without fail, Great Spotted Woodpeckers (regular daily visitors to the feeders so long as I'm nowhere in sight) will immediately fly away?? 

I'd have thought that, being fairly near the top of the pecking order, they would stay fearlessly around? I've even known Jays to stay on the lawn and keep feeding whilst I refill the feeders!

Any ideas why GS Woodpeckers should have reason to be so nervous?