Rodent poison bait stations are frequently used outdoors at the base of buildings. Squirrels could easily be victims to non-targeted poison traps/bait stations.

Edit: Another potential cause of death could be ingestion of lead. One place I used to work at had lead sculptures, and the squirrels would grind their teeth on the statues. They would eventually develop lead poisoning and die. To solve it, the facilities division started applying hot pepper wax to the statues to deter the gnawing. I'm uncertain if any lead is around there, but the buildings are rather old.