This same thing happened to me about a month ago, but my saw is a Chiwanese import, although most band saws have nearly this same design.
Behind the upper wheel there is a cast aluminum piece that the upper wheel axle is pressed into that tilts the upper wheel when you adjust the blade tracking. As you turn the tracking knob it pushes on the lower end of this casting to tilt the upper wheel. It only takes a small amount of tilt to change the blade tracking. My casting broke, letting the upper wheel tilt fully forward and the blade came off.
Since the original source of my saw no longer exists (Trend Lines), finding a replacement piece has proven a bit futile, so my son and I made a replacement out of 1/2" thick aluminum plate and then pressed the axle from the old piece into it. It's nice to have a son who is a machinist, certified welder, and seems to be a McGuiver type of person who can create metal things out of nothing, it proved quite easy for him. Being made from solid plate instead if a cheap webbed casting I don't think this will ever happen to my saw again.
Since Delta has discontinued stocking replacement parts for most of it's older products, I hope you don't have the same problem finding the part for your saw. If you dis-assemble the upper wheel shaft mechanism the problem, if it's the same as mine, will be very obvious. It's the piece that the upper axle is pressed into, and in my case the broken off piece fell out as I removed the back side wheel cover.
Charley