I don’t know. There are so many factors that can go into any one game’s eventual bundle appearance, both internal and external.
You can’t even say that games are only bundled after sales taper off, as we’ve seen games included in bundles right at or after release. (Botanicula was available for 1 cent in a 2012 Humble Indie Bundle that launched the same day the $10 game released. Other games have shown up in bundles while they were still selling well, often to the ire of early buyers.)
A game may be bundled because its sales have tapered off, and the dev/publisher wants to get a little more milk from that cow. Or it may be that the publisher is moving on to a new game, and isn’t just out to get a bit more milk from the old but also to try to win over future fans who might later buy into the new title. Or it may be bundled because it has built up some expensive DLC, and the publisher figures giving away the base game will spur DLC sales.
Or the dev may feel charitable, throwing a still successful game into a bundle as “fan appreciation” or to drive donations to a specific charity. Or the dev may be facing financial issues, and needs more money right now instead of later. Or they’ve read an article about how much money others have received through bundles, and decided to jump onto the gravy train without really thinking through the economics.
Or the dev/publisher is contacted only because the bundle organizer is putting together a theme bundle, and the dev has a game that fits the theme. Or the dev agreed to his game being in a bundle, but the contract gave the bundle organizer control over all the details such as the date, duration, or even how many bundles the game could be placed in.
Then there are the special case bundles. There are games that get bundled, but under the condition that only X copies can be sold and that the game will be pulled from offer if that count is exceeded. Unplanned “sweeteners” would also count, where an item may be added to prop up an underperforming high profile bundle.
And there are the out of left field bundles. The first THQ Humble bundle, Sony, Nintendo… Something unexpected changes, and the previous impossible is suddenly possible.
And through all this, we also have other general market changes. Bundles ballooned and then burst. Non-game bundles became popular. Humble Monthly appeared. Stores started giving away free games, then stopped. Now Epic is giving away free games. Now Epic is buying up timed exclusives. Devs keep switching back and forth between chasing the end of the rainbow on PC, console, and mobile. And contracts for cross-platform or even cross-store games can have pricing restrictions that affect when a game can be bundled.