Generally it is caused by either a blockage or partial blockage in the drain downstream. But I have known backfall on the drains leading away from the toilet to cause it as well, though this is rare.
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern B-C-FA--. That is, eight letter words with 1st letter B and 3rd letter C and 5th letter F and 6th letter A. In alphabetical order, they are: backfall
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 5 words with the pattern --CK-AL-. That is, eight letter words with 3rd letter C and 4th letter K and 6th letter A and 7th letter L. In alphabetical order, they are: backfall duckwalk hickwall kickball rockfall
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 3 words with the pattern --CKF--L. That is, eight letter words with 3rd letter C and 4th letter K and 5th letter F and 8th letter L. In alphabetical order, they are: backfall backfill rockfall
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 3 words with the pattern ---KF-LL. That is, eight letter words with 4th letter K and 5th letter F and 7th letter L and 8th letter L. In alphabetical order, they are: backfall backfill rockfall
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 5 words with the pattern BA----LL. That is, eight letter words with 1st letter B and 2nd letter A and 7th letter L and 8th letter L. In alphabetical order, they are: babydoll backfall backfill bankroll baseball
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 2 words with the pattern BA-K-A-L. That is, eight letter words with 1st letter B and 2nd letter A and 4th letter K and 6th letter A and 8th letter L. In alphabetical order, they are: backfall backhaul
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 10 words with the pattern B---F-L-. That is, eight letter words with 1st letter B and 5th letter F and 7th letter L. In alphabetical order, they are: backfall backfile backfill bemuffle bergfall billfold boatfuls bookfuls bowlfuls brimfull
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 7 words with the pattern BAC-F---. That is, eight letter words with 1st letter B and 2nd letter A and 3rd letter C and 5th letter F. In alphabetical order, they are: backfall backfile backfill backfire backfits backflip backflow
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern B--K-ALL. That is, eight letter words with 1st letter B and 4th letter K and 6th letter A and 7th letter L and 8th letter L. In alphabetical order, they are: backfall
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 7 words with the pattern B-CKF---. That is, eight letter words with 1st letter B and 3rd letter C and 4th letter K and 5th letter F. In alphabetical order, they are: backfall backfile backfill backfire backfits backflip backflow
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 18 words with the pattern B-CK-A--. That is, eight letter words with 1st letter B and 3rd letter C and 4th letter K and 6th letter A. In alphabetical order, they are: backband backcast backdate backfall backhand backhaul backland backlash backpack backpays backsaws backward backwash backyard buckrake buckrams bucksaws bucktail