Pretty crazy that Sonic Forces made the list. 

Sonic...fucking...FORCES!? O.o

I mean, I can sort of understand why... the other four games are games I could imagine people preordering and buying day/week one etc. and were all big sellers (Well, Shadow of the Colossus wasn't a huge seller but I'm sure it did more than enough), whereas Sonic Forces always felt like one of those "wait for a sale" kind of games, and it looks like all the waiting paid off for people, since they eventually got it for free, myself included! :P

If VGChartz software tracking was anywhere close to accurate, it also sold a respectable 560k physical on PS4 in the first 3 months, probably around 900k-1m with digital included:

https://www.vgchartz.com/gamedb/games.php?name=sonic+forces

Not bad at all. I'm not too surprised though, even though Forces didn't get great reviews, 3D Sonic always was pretty popular. Unleashed and Generations both sold over 5m across all platforms I'm pretty sure. 

It does kind of make sense if you think about it.
Most games that will be popular on PSN+ have already sold millions of copies. Sonic Forces was known to be a flop in both sales and reviews. So instead of having millions already having it, therefore not needing to download it, those people downloaded it because they are familiar with Sonic but didn't want to pay for it, since it was seen as such a disappointment to many people.

Edit: Ninja'd by @Bristow9091 

That's great news for SoTC , especially among the other juggernaut games and also compared to a game like Rocket League which launched as as PS Plus title, was downloaded millions of times and still didn't make the top 5.

Sonic Forces and Shadow of the Colossus were both recent additions. I believe they were offered the same month in the midst of the worldwide lockdown. People were home and PS+ activity was probably way up so people paid attention to the free games on offer. Also Sonic recently starred in a hit movie.

Huh, never realised the recent Sonic games sold so well, they're actually pretty decent numbers :o 

I have the top 4. Destiny holds the ignominious distinction of being the only PS+ game I rejected out of hand. As for Sonic Forces, it's sitting behind unnecessary elective surgery in my playlist.

Any specific reason why?