Speaking from my experience, it's not really FOMO. When I see a set I like, I usually wait till the last couple of days to join. With the exception of MoDoL, you gain absolutely nothing for joining early, so waiting till the last possible moment might be beneficial. You can see if the set is successful, if the kits you want will be made, if any other cool or maybe even cooler sets drop in the meanwhile and you don't freeze a substantial amount of money for a month. I appreciate people who join group buys early, but it doesn't surprise me at all that the last couple of days have huge spikes.
As for this group buy, congrats to kema for a massive kit count and quite possibly the best selling desk mat ever.