1. Some large-userbase OS+messaging services provider, like Apple or Microsoft, sends a “proposal” for some new codepoints to the Unicode Consortium. The explicit message is “do you think these are worth standardizing?” But the implicit message is more unilateral: “we’re giving you advance notice, that we’re planning on including this emoji in our fonts + supporting it in our messengers, whether you like it or not, and whether it’s inter-compatible with anyone else’s systems or not. If you don’t standardize it, we’ll encode it using the Private-Use Area.”
2. Given the Unicode Consortium’s goal of never having proprietary text bits flying around the Internet — and given these big service providers’ histories of running messaging services (e.g. MSN Messenger) that did use proprietary encodings for emoji, resulting in some lasting problems with digital archaeology — the Unicode Consortium feel pressured by this “proposal” to standardize the proposed emoji codepoints, whether they really think they’re “worth” being standardized or not.
3. The Consortium also then feel pressured to get a new standard revision out quickly, to get ahead of the planned usage by these service-providers (because that usage would have to be encoded somehow, and if they don’t give the service-provider a codepoint to use by the time they ship their new font version that includes the relevant emoji, they’ll just have to make up their own.) In most cases, the Unicode Consortium is reactive to existing usage, allowing them to judge whether there is real existing adoption of a symbol; but here, they have to be proactive, pushing out a standard that includes a codepoint before knowing whether anyone will use it.
As a result, you’ll see these large dumps of new emoji where Apple/Microsoft/Google/etc. just decided autocratically that the world needed some more emoji, and Unicode begrudgingly followed along. It’s like what happened when Unicode first absorbed the emoji codepoints of Japanese feature-phones; but happening just-in-time, one at a time. (If those Japanese feature-phone manufacturers kept introducing new emoji, they’d have had to absorb those just-in-time too.)
There’s a separate, much more sensible and deliberate process the Unicode Consortium goes through you’re a regular-sized actor who’s not attempting to strong-arm them, where you do tend to have to prove existing use in either analogue documents, or as part of some proprietary digital text format.