What would be causing Christmas cake to crumble on cutting?

The amount of sugar added in the recipe should be checked as excessive amounts may cause cake to crumble.
The role of sugar in a cake recipe is to tenderise, moisturise and aerate as the fat, usually butter, and sugar are combined in a process called 'creaming'.
Sugar attracts the moisture in the recipe, drawing it away from the flour and preventing the formation of gluten. Sugar also tenderises by slowing down the coagulation of the egg, flour and milk proteins that set the structure of the cake when baked.
However if too much sugar is present, the cake is so tenderised that gluten is unable to maintain the volume of the cake and it begins to crumble when cut.