"Now it was the practice of the priests that, whenever any of the people offered a sacrifice, the priest's servant would come with a three-pronged fork in his hand while the meat was being boiled and would plunge the fork into the pan or kettle or caldron or pot. Whatever the fork brought up the priest would take for himself. This is how they treated all the israelites who came to Shiloh." NIV translation
Here, we see that the entire priesthood at Shiloh had become corrupt. In Leviticus 7, we see that God gave certain parts of the offering to Aaron and his sons as their portion.
Here, we see that the priests did not follow God's instruction but instead had developed their own system of determining their portion. They simply believed that, if God did not stop them from taking a portion, it was acceptable to God. They developed a tradition and it took the place of the very word of God that had been given to them.
Even though we do not offer animals as sacrifices, the same type of thing happens today.
It is easy to fall into this tradition trap even when it is pointed out that it goes against the revealed word of God.
"But even before the fat was burned, the priest's servant would come and say to the person who was sacrificing, 'Give the priest some meat to roast; he won't accept boiled meat from you, but only raw.' If the person said to him, 'Let the fat be burned first, and then take whatever you want,' the servant would answer, 'No, hand it over now; if you don't, I'll take it by force.'" NIV translation
Eli's sons were even more corrupt than the rest of the priesthood as they did not even wait for the meat to be boiled. They were wanting to roast it instead of having boiled meat. The boiling was to remove the fat and the blood which was set aside for God so, by insisting on taking the raw meat, they were taking God's portion. We also see that they had servants to do the dirty work for them and they would even threaten the people if they would not give in to them. The priesthood had become a bunch of lawless thugs preying on the people.