The Liberals had a minory government, meaning they had the most seats, but not enough to win a vote without an alliance with another party. This has happened a few times in Canadian history, and arrives from us having (right now) four main political parties, rather than just two like the U.S.
Recently, the Liberals have come under scandal (well, the scandal's old, but it's official now), and the other parties have been waiting for the chance to bring them down. They introduced a motion saying that they no longer had confidence in the Liberal government. The motion was passed, because nobody was allied with the Liberals, and so we're back to the polls for the second time in 18 months.