I am sure in this case it was stress that caused the seeds.
However, pollination can also occur without any mistakes, stress, or strain problems. I was reading an article a few years back that the pollen count in New York at that time was 70% marijuana! Pollen can travel many miles, so it is possible to get a seed here and there. Where I live there are most likely a lot of "gorilla grows" (outdoor in the woods and fields). Usually in the fall I will get a seed here and there. I presume it is from those outdoor grows.