Toriel has made a commitment to herself, that she would not let any human cross the gate that gives access to where the other monsters live to or protect the human from the monsters harming him, or you harming them... But now it seems that she came with new ideas to prevent any human from leaving her house, by eating every human that falls in the underground... Starting with Frisk.
After digesting Frisk, Toriel had an idea that will make him more useful than just making Toriel's belly soft and plump, she can use the humans souls that she will trap inside her belly by eating them, break the barrier and leave the underground... So she can look for more tasty humans to eat and get more powerful~