bhomburg

There is a lot of manual paperwork involved when you check a dog as baggage. Seeing these are shelter dogs where the passengers checking them in don't know the procedures or the numbers (a seasoned passenger can tell them "my dog weighs 78 pounds including the kennel or "it's 11 points" or even "no need to fill all this out again because it's still attached to the kennel from the last flight" which speeds thing up considerably) this can easily take twenty minutes and more per animal even at, say, DFW.
Combine that with the general situation described by usviflyer above and the "frequent flier" in line behind the group getting upset is understandable.

edit: There is limitations in place on how many dogs can be taken on any given flight already. The compartments only hold a finite number of crates. I once had to rebook and wait one day when trying to fly from MXP to JFK as there were a bunch of dog show attendees checking in dogs who turned up at the airport earlier than me and there was no more space for my dog.