Undereducated farmers motivated by greed with unlimited access to machinery and strange drugs- and not just in China. "Relax. The world is perfectly safe perfectly safe perfectly safe. Share and enjoy."
Cucumbers and watermelons come from wild cucumber plants in the past and the way wile cucumber spreads it's seeds is to explode. I first noticed one when I was in Greece, walking through a field on way to a taverna. I saw what looked like a watermelon plant and walked over to see it better. On it were a bunch of small melons about thumbsize that looked just like minature watermelons. I bent over to pick one and just as I touched, it exploded, spraying me with it's sticky sap and seeds. Anything jostles the plant when the fruits are ripe and they explode, sticking seeds to the animal, who then has them drop off often far afield. Some people react to the sap same as poison ivy, although I think the agent at work is different. I managed to pick a few that didn't explode and asked the taverna woman what they were. She got upset, started shooing them away, had me toss them and wash the juice from the plant off me. She was sure I'd break out, but next day still no problem, which I showed her when I stopped by for lunch the next day. Nevertheless, I left them alone after that, just to be safe.
Here's a picture that's closest I can find to what I saw.
Ah, I found a video. These are barely ripe, but they do explode for the camera.