A downstream blockage or partial blockage of some sort. As the WC bowl fills the water is unable to run away quickly because of back pressure.
I suffer this two or three times a year - for me the outside inspection pit where all the drains meet develops a blockage in the outflow pipe. With experience it now only takes me 10 mins to clear it using a length of plastic pipe.
If you have an inspection pit then prise the lid up and take a look before doing anything else - it's an easy check to do.
Good luck...
Peccavi, August 2009
