I think it has to do with the summer months coming up and people getting ready to move or go on vacation. When I worked in a large, regional shelter there was always an upswing in surrenders and "strays" during these months.
It starts with the older puppies who have been living outside for a few months now and the neighbors are complaining about the barking, or the yard is getting torn up by the 9 month old puppy that was quiet and "gentle" as a two month old. The kids can't play in the back yard because the untrained puppy is now big enough to knock them down and scratch them up in a frenzy of excitement at finally having someone in the yard.
Who wants to pay to board a dog for the two week family vacation when nobody "plays with it anymore"?
Same goes for people moving during these months. The kids are out of school, which is a great time to move with a minimum of disruption. Who wants to pay a pet deposit on a rental, or keep looking for a home to buy that has a great fenced yard, is pet friendly and is in a good school district/close to work/whatever when the kids haven't walked the dog in years?
Then, starting in late September, the surrenders start of all the dogs that were purchased during the summer months while the kids were home from school. Once school starts up there is no time to play with/exercise/clean up after a dog. The kids are the ones who asked for it to begin with, and they just can't keep up with the responsibility.
In and amongst the surrenders are the strays. They are either turned in as strays by owners that don't want to pay a surrender fee, are too embarrassed to surrender outright or they get loose and the owner figures that someone else will give them a "better" home and so never look for them or make a lost report. In many cases, a missing pet is seen as a relief and just one more item to be crossed off the To Do list ("well, I was going to put an ad in the paper and find a new home anyway").
The cycle slows down about mid-November. Just in time for a rush of purchasing that is the Christmas season, which in turn supplies the crush for the next year of surrenders and strays.
Add to this regular cycle the depressed economy and increase in home foreclosures and the picture gets even more bleak.
Sheilah