In this case "spreading" means "happening more often". And you could have a stable rational market where power prices go negative for an hour every day.
It's spreading because the market is oversatured with renewables because of government subsidies. A sound investor would stop (or go bankrupt) when the negatives outweigh the positives.
Subsidies seem to be on the decline. In fact, I was just reading that due to the Chinese government cutting back, people are predicting PV prices will plummet in the next year.