Trick Shot can bounce between up to three enemies. Do each of these hits have their own check for critical hit? Will on crit effects such as sigils or traits trigger multiple times for one skill usage?
Any skill that hits multiple times or hits multiple enemies can crit on each hit.
Additionally, any crit can cause on-hit effects, regardless of whether or not it is part of the same attack or not.
This is why a Greatsword warrior is good at proccing "on-crit" effects -- 100 Blades hits 8 times over its 3.5 second duration, which means that on-crit effects can trigger multiple times (though most sigils have a cooldown period before they can be triggered again).
A fire elementalist MH dagger is also good for a crit build, since they get 3 claw attacks every .5 seconds.