refuses to go home

Question
We adopted an 8 month old lab mix not quite two weeks ago and things are
going quite well.  But we are encountering a problem we have
no idea how to deal with.

She loves walks and we take her out often.  But
sometimes (although it seems to be happening more
frequently) when we are on our way back to the house,
she starts to really resist coming with us.  She
starts chewing the leash, jumping up and biting at
anything she can get her teeth on or laying down
refusing to move and biting at her harness.  
We don't know why she is acting this
way.  She gets out often and even acts this way after
an hour walk.  She seems really happy with us and we
don't use any punishment with her, only positive reinforcement.  There is no
reason
not to want to be in the house except  that
she doesn't like when we leave her alone to go to work
and school.  We use a crate and provide toys and treats for her.  
What do we do when she behaves like this?  

Answer
There are several possibilities that I can readily think of.  First, she is right at the age where pups go through their second chewing phase.  So, she will be seeking to put things in her mouth and chew.  Second, she is also at an age where dogs test limits - such as "hey, wait a minute, I wanted a longer walk".  Third, your assessment that she has learned that going back to the house is the first thing in a sequence that leads to her being left alone is probably pretty accurate.  Dogs learn in pictures!  I'm glad you are using positive reinforcement, and you can use it for this, too.  http://taylor.uurnet.net/pipermail/click-l_click-l.com/2004-April/000216.html
I would be thinking in my little trainer head, leave a bit earlier for the walk, use this method, and you can see that your dog gets an extra bit of a walk, or some play, for exhibiting "no leash grabbing".  
You can also change the pattern - walk earlier in the morning, and then go about some other chores long before it's time to crate her and leave.  Also, be sure her crate is a haven, not a prison.  Perhaps you can enrich the crate environment by leaving her a Buster Cube, stuffed Kong, or other interactive toy.