Strawberry pretzel salad. Crust probkems

The most likely cause of the hard crust was due to over baking your crust in the first step. Pretzels are previously baked dry and hard, and the usual mixture of butter and sugar isn't bringing much moisture to the party. Between the sugar and the pretzel, you risk creating a sort of pretzel brittle- great as a candy perhaps, but not so great for a crust. If your oven temperature was a bit high- that can also cause troubles, so it might be worth double checking your oven thermometer accuracy. So it was either a problem with the heat being too high, or being in just a little too long.