So all of them even those who were not with the GW and far away were killed by some magic?how did it spread?
If it was anything like the blight, then it could have been done by exposure to corrupted blood or infected tissue. In which case, it would imply a certain amount of physical contact (which fighting tends to do).
It is assumed that the wardens knew and were in possession of all (or almost all) Griffon breeding grounds and their habitats, and since there was no way to move or relocate these breeding grounds, a disease running rampant to such an isolated population may very well wipe them out if the joining was introduced to them.
Somehow I don't think the wardens thought about doing an isolated "test trial" before they started mass producing these tainted griffons. Very likely the travel and use of these tainted griffons going over to other Warden outposts and their corresponding breeding grounds, is part of why it spread as much as it did.