Several mutations may be needed before the cells actually become cancerous, and it may take some time for each of the needed mutations to accumulate in the same clone of cells.
Cells break through the basal lamina and spread into the muscle layer.
They have mutations in genes encoding proteins that are used to repair DNA mismatches. Colon cells may be exposed more than other cells are to mutagens that cause base pair changes requiring repair by the mismatch repair system. They may also have different regulatory makeup from other cells that makes them especially susceptible to such damage. For example, they may be less good at halting the cell cycle before damage is repaired than other cells are. Finally, they may divide more than other cells, which would provide more opportunities for mutations to be passed on to daughter cells.