Small random changes yes, but long specific sequences become exponentially less likely. People on the other hand can decide to add say the first 100,00 digits of Pi or some other marker for internal use. Picking a useful sequence is hard, but we have the technology to add any specific sequence as long as the plant can survive it.

So it’s not just a question of random chance adding something harmful at this point, but also malicious actors. Someone could easily decide to try and reduce the human population by reducing fertility or whatnot and new GMO crops aren’t tested for such things.