Mucous builds up as the diaphragm muscles start to weaken. The more they weaken, the more difficult it is for PALS to cough out or fully swallow excess phlegm, so it can accumulate in the lungs.
For secretion, as the bulbar muscles weaken, excess saliva can occur, again, difficult for PALS to get rid of. As Lisa mentioned, certain foods can trigger more saliva and more mucous.