I have a chair for my outdoor table on the deck that has all of sudden
decided to go back WAY too far and nearly tips over. Any suggestions on
how to either fix it or McGyver something so it won't kill people?

Some of the new chairs use two rectangular blocks of metal instead of
more traditional springs. They are like a single layer leaf spring.
If one of the two is broken or has become disconnected, it might not
be obvious when looking at it without someone sitting in it try to
lean back.