Space shuttles use a combination of super-cooled liquid hydrogen and liquid oxygen as their fuel. This fuel is much colder than the temperature of the air surrounding the shuttle and the difference causes water vapour in the air to condense on the outside of the fuel tank.
This is similar to condensation which forms on the outside of a cold drink can when you take it out of the fridge. As this water vapour collects and cools further it freezes into patches of ice and the intense forces during launch cause the ice chunks to fall away.
Answered by Rik Sargent