Why do the townspeople line up to fight Odysseus?

The correct answer is - They are angry that he killed all of Penelope's suitors.

When Odysseus came back from his long trip, he saw that his house was filled with men who wanted to marry his wife Penelope, thinking that he was dead. He grew angry at this, killing all of them in order to secure his marriage. Obviously, killing all nobles in Ithaca and beyond angered townspeople, who wanted their revenge.