The problem is starch. And potatoes contain a lot of starch!
Mashing cooked potatoes gently by hand or with a ricer leaves most of the starch molecules intact. The butter and dairy you add to the mashed potatoes are able to coat each individual particle, making the potatoes creamy.
But the quick-moving blades of a food processor will actually tear the starch molecules. The released starch mixes with the liquid in the cooked potatoes, and the mash transforms into a gummy paste before your eyes. Highly unappetizing.
This can happen in a matter of minutes, so don't even be tempted to use a food processor to get the last few lumps out of your potatoes!
What are your tricks for getting good mashed potatoes?
Related: How to Pick a Potato