I too believe that it’s mostly cancellations but I experienced a time when almost every ride dropped at once. I went on Saturday December 28, a day that hit max capacity and they stopped selling
Disneyland Park tickets. Refreshing maxpass basically made a stressful trip into an enjoyable one. Some of the notable grabs are as follows. All were for a party of 5 or 7:
8:30 - Indy for a half hour out
11:00 - Space Mountain for an hour out
1:00 - Matterhorn for 5 minutes out (yes really)
3:00 - Big Thunder for a half hour out
4:30 - Haunted Mansion Holiday for a half hour out
6:00 - Big Thunder for a half hour out.
Two days later I was getting Guardians, Soarin, and Toy Story at a half hour out easily throughout the day. This was on December 30 with ridiculous crowds. At 3pm, I snagged a 7:30pm Radiator Springs for 5.
But the thing I remember most was getting those big drops with 7-8 rides all showing up at once. It didn’t happen very often, but I certainly remember it. Most of the time Grizzly Rapids was just showing up. In fact that ride was keeping the app from saying all fast passes were distributed for CAAdv. Kept refreshing for 10-15 minutes, then BAM a bunch of rides showed up at once. This is why, in my anectodal opinion, I believe there are drops. BTW, last year I took advantage of the known Toy Story Land Drops at Hollywood Studios, and boy oh boy was that awesome. So I was indeed looking for them at Disneyland.