Rengar just did 2238 dmg with a basic attack.

Cause you never upgrade your trinkets.

The lack of context is delicious.

what were the factors?

You sure it was a basic attack and not his Q?

Probably his Q, which is an Auto Attack modifier.

Animation cancel. Multiple basic attacks add up on the death recap, so he probably made two or three AAs with animation cancels.

Where is your armour item like if you complain when you go full glass canon against a fed rengar you deserve to be blown the fuck up like even an omen will negate most of that damage. Oh but more damage is better is it really now if you can't get a single auto attack off?

IKR THE FEELING.. Yesterday I played against a rengar that killed me in 0.15 sec everytime he jumped on me.. Pink wards helped sometimes, but when I didn't have them I just didn't have any time to use my zhonya. Because you can't react in 0.16 sec

Balanced

You probably got that from death recap. That just means that all of his basic attacks combined (not just one) in the time interval stated in the pop-up did that much.