Those are existing segments created by other cyclists. As we approach and exit segments Strava highlights them for folks who want to try for a personal best effort.

It's one of the cores of Strava. I'm not sure there's a way to disable that. I kinda wish people would disable it for group rides, which turn into free for all sprints that scatter the entire group.

If it bothers you, try another app like Wahoo Fitness, which is free, low resource and more reliable than Strava for recording rides. Then upload the data to Strava. That's what I usually do. Not because the segment notifications bother me, but because Wahoo Fitness is more reliable, less drain on the battery, and interfaces nicely with Strava afterward.