What causes a shad die off. Went fishing and dead shad was all over the lake and banks. Needless to say,fishing was bad lol!

Water temperature getting colder than 45 degrees or a sudden water temperature change of more than 10 degrees. Old age also kills them off, but not in mass numbers.

Tom

i would of collected some of the fresher ones and froze them for catfish bait later on

Water temp, forage, lack of oxygen ? 

It's a free meal for any fish then. Even the true predators that like live bait will get the ones that are barely hanging on. It can get tough, but fish definately know what is going on and will be somewhat in the area.