Something else, called being cheeky. It's also potentially dangerous and is certainly undesirable unless you want to either have your toes crushed, be pulled off your horse or have your horse get a tooth caught in the iron. I would suggest a swift kick in the nose whenever she tries to do this.
I've had this done to me before but it was more of " can you get off please, I want to poop"
haha, so she may be trying to tell you something or as others have said just being cheeky.
Blacky does this too, but she just nudges my feet doesn't bite them.
As midi says, often when we get back from a hack she's been needing the toilet but other than that I've never worked out why she does it!
I Hardley know of a horse that does not do it!!! Usually when they think they are being ignored !!! Rude of them but I find a little chat often stops it, if all else fails a tap on the nose soon stops it
Star used to do this - until one day, I moved my foot 1" forward as she brought her head round......with the inevitable result. I didn't kick her in the teeth - she banged her mouth on my boot toe....and she didn't do it any more.