All the horses I've ever worked with have done that to one degree or another. It's like they're checking out where those cues come from. I let them sniff or nose my foot as much as they like, but no chewing allowed!
My gelding does this when new people are on him and he's not sure, he will turn and smell there feet...very interesting but I don't think its anything to worry about. That's really the only thing they can reach when your on them, might be a way they are still making sure its you up there or someone else who has interesting boots....lol
My girl checks my feet out every time she sees me whether im riding her or not and sometimes she trys nibbling I can pop her and she just comes back for more. I think they are checking out the smell of SHOES maybe its a sign our shoes smell lol.
Ahahaha, my off-the-track standardbred mare does it aswell lol, and this horse I used to ride a little welsh pony, well he would turn around lick and suck on the end of my shoes, maybe its just something that makes them happy LOL :P