This isn’t a random weed that blew in from a neighbor’s yard though that does happen. Long Island has over 100 golf courses, many of which use bentgrass on their greens and fairways. Seed migrates. Wind, water runoff, and equipment carry it onto adjacent residential properties, and once it establishes, it doesn’t leave on its own.
There’s also the housing stock to consider. Much of Suffolk County was developed in the 1950s, 60s, and 70s, when lawn seed mixes commonly included bentgrass varieties. Older lawns in Centereach, Selden, Lake Grove, and Ronkonkoma may have had bentgrass growing in them for decades spreading a little further every year.
Add in Suffolk County’s sandy glacial soils, which allow stolons to travel easily through the soil profile, and the county’s maritime climate, which extends the cool-season growing window that bentgrass thrives in, and you have conditions that are genuinely favorable for this weed. It’s not bad luck. It’s geography.