Damage occurs all at once, thus, very technically, your contingency will not activate
The rules on "Damage and Healing" state:
[...] Whenever a creature takes damage, that damage is subtracted from its hit points [...]
It doesn't state that the damage is dealt 1 hit point at a time, it is all at once. This means that you will immediately drop to 0 hit points (as 5e doesn't have negative hit points). This technically means that contingency won't activate because you never dropped to 1 hit point, you dropped to 0.
Another thing worth remembering is that contingency always activates after its trigger:
[...] The contingent spell takes effect immediately after the circumstance is met for the first time, whether or not you want it to, and then contingency ends [...]
Even if you have it trigger when you reach 0 hit points, you will still briefly fall unconscious
This is true because contingency occurs after its trigger, and thus you would have reached 0 hit points and as the section on "Dropping to 0 Hit Points" explains:
When you drop to 0 hit points, you either die outright or fall unconscious [...]
Ultimately this means that you, for an instant, will fall unconscious (assuming you don't instantly die from excessive damage) and thus:
The creature drops whatever it's holding and falls prone.
Compare this set-up to the death ward spell which states:
[...] The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends [...]
The death ward spell will instead cause the creature to take all 30 of the damage, and then end up at 1 hit point, never falling unconscious.