I've been asking myself the same question.
The addon I maintain gets updated very regularly, sometimes every day or at least multiple times a week.
I've seen major/minor releases with thousands of lines changed be approved instantly while others with only 1/2 line changes (bugfix for example) taking an hour: https://www.curseforge.com/wow/addons/gargul/files/3453937. This leads me to believe that there are no manual steps involved in the approval process or them being taken with a grain of salt at the very least.
I've also uploaded the same addon for two versions recently (for 1.13.7 and 2.5.2), with the 1.13.7 version being approved instantly and the 2.5.2 (aka the more important one) taking hours even though the only change was a number in the toc file. At some point I archived and deleted the 2.5.2 version, uploaded the same one again after which it was approved instantly.
I'm grateful for CurseForge and similar platforms and all the effort the maintainers put in it don't get me wrong, but the last thing you want as an add-on developer or distribution platform is updates (that potentially fix crucial bugs) being stuck in limbo for no apparent reason.