Trousers shrunk at waist

its a strange situation. Till other day, every other trouser was fine. But lately, I realised that all the trousers which were of waist 30 have shrunk and now are NOT fitting in me. One day early morning I got late for office as all my 30 sized trousers have shrunk so much that I had to borrow one from Boi. That was the trigger which resulted in an expensive weekend. So on this weekend I bought a couple of trousers with waist 32, so that even if they shrunk, they fit in me. I know many people are facing similar problems with their trousers too, but they dont know the exact root cause :) :)