I don't really know, but I think she might have met a dog she WANTS to mate with, but since she's neutered she might just be satisfyed with pretending.
Both my huskies (male and female) find/make "dens." They often will relocate to another part of the house or yard until it's time for them to relocate again.
I had a female that did this. She would take her toys to her "den". She allowed my husband and myself to approach her being submissive to us, but not the children. She was not neutered and this happened during her heat cycle. This was not a good thing. Eventually she bit my youngest, (not during her heat cycle) which required surgery. We had to give her back to the kennel from which we bought her. When she arrived back to the breeder, she growled at everyone in the owner's family and was eventually put down. The breeder traced her entire "family" to find that the females (her mother and the female puppies) had all done the same thing and had all been put down. There was not a problem with the males. The kennel owner had loaned out her male as a sire--we got the pick of the liter. The kennel owner never did this again without more info on the line of dogs. You might just have bigger problem---check out the rest of the dog's history and family line.
I have a 4 yr old female who does this too....she is in "False Pregnancy" Its starts about 8 weeks after she finishes her heat cycle she begings to collect her toys (very specific types of toys..rubber squeeky ones) and she will carry them around the house, from room to room until she finds a place to nest. The last few times she went through this she also lactated from her nipples. Now my other husky does not go through any of this after her heat cycle, but I did have another husky when I was a child that would do the same thing, but she was spayed.