They saw some of his disciples eating food with unclean hands. (They were eating without first ritually purifying their hands through washing.
The Pharisees and all the Jews don't eat without first washing their hands carefully. This is a way of observing the rules handed down by the elders.
Upon returning from the marketplace, they don't eat without first immersing themselves. They observe many other rules that have been handed down, such as the washing of cups, jugs, pans, and sleeping mats.)