One would expect an incident like this to happen in Alphys's lab. This wasn't usually an at-random phenomenon, this looked like one of the abnormal side-effects of an experiment Alphys would conduct. After all, she was known for causing weird and strange effects.
However, it was completely different. The cause was unknown. The condition was never seen before. And Alphys wasn't even near Soda the week before it happened.
Asriel just woke up to see multiple... Sodas?