When Obadiah announces Bobby's death in the kitchen, Susannah thinks about all Mrs. Shandy's clothes that she'll have to dye black for mourning.
Obadiah hopes it's not true, because they'll have a heck of a time convincing Mr. Shandy not to improve the field.
Trim seizes this opportunity to sermonize a bit about the fragility of life. And Tristram interrupts to remind his readers that we're all nothing but bodies with senses before he returns to Trim's speech.
He punctuates Trim's speech by dropping his hat on the floor to symbolize mortality—a very effective move.