why?

im not sure if this is a problem or just a characteristic but
each time i bring my dog inside before sitting down he always
puts his head to the ground in nearly does a full roll
before he lays down...he cant just sit down normally
im just wondering if there could be a specific reason for this

[Jim Humphries, B.S., D.V.M.]

Probably just something he has learned to do.  Does not indicate a problem that I know of.  I've seen dogs develop many interesting and highly unique behaviors.  I'd love to see the video!  

If you wanted to try to change the behavior, you would need to distract the dog with food or fun just before he began the behavior, then reward the new behavior.  Over time he will begin to look for the new reward and forget the old behavior.  

Does not sound necessary, but if you'd like to modify it, that's the way I'd try.  Good Luck.