It is git in general. This answer suggests, it is because of the filelocking. For more info, just search SO for "git samba"
From my experience, operating on a non-bare repo on a network drive is a bad idea.
For best-practises: http://git-scm.com/book/ch4-1.html
So either:
- Setup an SSH server on you NAS and push/pull to that
- Map your network share (as you have now) but put a bare repo on that. Then, push/pull to that one.
In any case it is recommended that the git repo with the working copy resides on a local disk.