When does a game go on a bundle list

TL;DR: Just treat anything as bundled, even if it is on the list at the current moment or not.

Essentially, if anyone in the world at any point in time can get the game for a price of ~5% or lower of its non-discounted US Steam store price. Mostly it means bundles, but price glitches, extreme normal store discounts can also trigger this.

The 80% thing is a bit of a misunderstanding: since the Russian Steam store carries an inherent 40-80% cheaper price range for most anything (to combat the rampant piracy, not because the Russians are poorer than, for example, Venezuelans or Vietnamese), it means if a game goes on a ~85% sale, their lower price range will make the game cost so little that it reaches the threshold.
(Simply put: if a game costs 10 dollars in the US store, its Russian base price is likely around 4 dollars. If the game gets a 85% discount, the US store price will be 1.50, but the Russian one is around 0.60, which is enough to add it to the bundle list, because, you know, paying 60 cents for 10 dollars of CV is a bit exploitative.)

As for when, it can take anywhere from 20 minutes to 20 months. We have seen some extreme backdating for a few more exotic price glitches. There have been amazon price glitches that affected a whole one country for like 30 nanoseconds, but people got enough keys to exile the game to the bundle list.

As ppl said ^^
Also game could be bundled when it was free
for example, Woodle Tree 2: Worlds - keygiveaway from dev for positive reviews,

abusing CV with setting shady price for cv farming, like viewpoints dev did

and more.. ¯ \\ (ツ) / ¯