Moving houses?

[xXHazelnutXx]

My young dwarf hybrid hammy Zelo has recently gotten into an odd habit of moving his bedding from both his current houses (one in each of his joined cages) to the top floor of one of his cages (see image below) now its not a major concern because I know hamster have a nest building habit but I'm trying to work out why he likes his up there. Odd question I know but any ideas?

http://i743.photobucket.com/albums/xx72/xXHeizeruXx/20121204_180951.jpg

[Stockannette]

Flower did this several times. I wondered if she liked the top floor better in warm weather or something. I'm marking this thread to read what the experts say.

[KDHamsters]

My hamster did this its normal.