The toilet rim holes and the siphon jet hole may clog up up over time depending on water conditions. Often people put things in the back of the toilet tank such as that blue stuff that helps to clog up the holes. If calcium builds up it is difficult to remove and can be dangerous. For the toilet to flush properly water has to flow freely through these openings for a good flush.
1)It usually happens very slowly and over time. I think you just noticed it at this time. But while you were working on it a section of slime may have sloughed off and got stuck in the siphon jet.
2)It does happen more often if you use that blue stuff and if you have hard water, scale can build up over a long period of time. This scale can be removed with acid but most people just get a new toilet in that case. Some people just get a new toilet because they don't know it can be cleaned out or don't want to do it.