Not all that weird - there are a number of weapons that have the special ability to be used as tripping weapons. Generally, these are special curvy weapon, chain weapons, and other weapons that are designed to hook around your opponents' ankles.
From the PHB, you've got: bolas, spiked chains, dire and normal flails, guisarmes, halberds, gnome hooked hammers, kamas, scythes, sickles, and whips (as well as any newer weapons in the expansion books).
Some of them even give bonuses on your trip attempts.