I didn't do statistics, but The Aviation Herald lists units to low tens a year worldwide.
Most of them happen at airport due to fire indication (sometimes even false) or smoke on board.
On the ground, fire spreads a lot faster than while flying, because when flying, the wind is too strong and blowing the flames off. The most notable example would be China Airlines B738 at Okinawa on Aug. 20th 2007, airplane in flames arriving at stand. The aircraft was already at the gate when the flames were first noticed, evacuation was immediately initiated and by the time everybody left, it was already engulfed in flames, about 3 minutes after the fire started. While China Air might not be "western" in strict sense of the world, it is largest airline on Taiwan and follows the same standards.
Another notable accident is Egyptair B772 at Cairo on Jul 29th 2011, cockpit fire.
Here are some recent evacuations mentioned on AvHerald:
Notice, that they are all smoke or fire events. For some other reasons, see e.g.:
two similar accidents where both aircraft were evacuated via slides. The reason in these, and many other gear collapse incidents, is that they don't want to wait in a damaged plane where there is a risk of fire, plus the difficulty of getting stairs to the aircraft resting in strange attitude.