They can ingest the fleas if the sniff/munch on any dead (or live) things that carry fleas. Squirrels, rabbits, etc found in the environment are the most likely cause. Since you are already giving an oral flea preventive, get a script from your vet (assuming they don't carry it) for Sentinel Spectrum. It prevents heartworms & fleas, and kills rounds, whips, hooks, and tapes.
ETA: You can find it online for about $11.25/monthly dose for a 50-100 lb dog.