Trip is a character from the game Façade. It was top priority to annoy him in anyway possible.
Instead of actually finishing the game, PewDiePie would occasionally "take a break" and go annoy Trip for a while. Trip seems to exhibit an exaggerated aversion to the word "melon". When PewDie uses that word against him, Trip throws him out of his house without further conversation.
Adding on to Trip's hatred to melons, PewDie was playing and he happened to say "melon" and Trip did in fact throw him out.
Trip's hatred for melons is unknown. A theory is that Trip may take the word Melon to a synonym of breast, or that he may downright despise melons as if he's tried one before and found it incredibly disgusting.