20 Sept 2016
For the ability to trigger, Depala has to actually change from untapped to tapped. If an effect attempts to tap Depala, but it was already tapped at the time, this ability won't trigger.
20 Sept 2016
Depala, Pilot Exemplar's last ability is a triggered ability, not an activated ability. It doesn't allow you to tap Depala whenever you want; rather, you need some other way of tapping it, such as by attacking.