How can the plane be too heavy?
 

My husband commutes from Spain to London on a weekly basis, usually with Easyjet.

A couple of times recently the plane has been delayed because it has been too heavy. The catering trolleys are then unloaded to lighten the load.

This means that not only is there a delay, but there are no refreshments on board.

I am wondering why this happens?

Surely they must know how much weight has been checked in before take off? Presumably they allow so much for the weight of passengers and hand luggage, so why do they allow too much extra weight to be checked in? I know there are several commuters who don't check luggage in at all!

The maximum weight can vary with the weather conditions, and the current weather might not be known until close to takeoff. For example, if the wind direction shifts, it may be necessary to take off from a shorter runway than usual, which could reduce the allowed weight. I was once on a flight where this situation came up, and the airline chose to add a fuel stop rather than offloading anything from the plane.