No surprise really. Sri Lanka had 2 world class batsmen in Sangakkara and Jayawardene who were the glue that held their middle order together for years. Now without them both, the rest of their batting is brittle, especially against the England seam attack in English conditions in early summer.
It was a great knock from Moeen Ali earlier. He rode his luck and against a team that bowled better and was not so dismal in the field he would no doubt have been back in the hutch long before he made his hundred. But he played some exquisite shots and he can surely only grow in confidence after a big score like that.