All the states where these protests are happening have been Republican-controlled for a long time. This in turn means that they've gone out of their way to gut infrastructure, public spending, etc., including on things like education. And now it's coming back to bite them in the ass, because teachers are finally sick of it.
The situation in Oklahoma is a good example. Education funding was cut by 30% over the last decade, schools can't even be maintained, and teachers are drastically underpaid. Republicans have made a religion out of cutting any and all government spending, and they're finally paying a price for it.