Did anyone just get random Sanctum 2 cards?

No but that sounds like a booster pack. You can get random packs for stuff you qualify for.

Sometimes you get that when cards are introduced for a game that didn't have cards. If you have played enough time in that game, you should get the cards after some time.

I just randomly received some Killing Floor 2 cards about 15 mins ago

edit: i'm still getting cards.. a total of around 20 so far

I got a single Castle Crashers card while playing something else...

My friend got a Sanctum 2 card while playing Borderlands, meanwhile I got two Endless Legend cards while doing nothing.

Here is a bizarre theory, maybe your account had restricted card drops but that restriction was lifted, since you have 2.1 hours of playtime that would be enough for the 3 cards to drop and that is the new way card drops happen.
Edit: Didn't read you already had the drops, looks like a steam glitch then :)

I got a card from Stellaris and one from Dirt Rally. Haven't played both games for a longer period of time.

Maybe they expired from the marketplace and returned to your inventory. I think there is a time limit if they don't sell.

I got a random rocket league one.

I just got two Arma ]|[ cards. Haven't played it for a year or so.

[edit] Just got two more, one Tropico 5 and one Dirt Rally.

Maybe they are adjusting some card drop rules and handing out the ones that we didn't get by the old rules but are entitled to by the new ones.

[edit2] and one more from Dead Island DE

all the cards seem to come from "remaining drops" from the games that I played for a short time, got some cards but not all; so nothing really extra.

Just got one for DiRT Rally.

I previously got just one by playing for an hour some time ago. And it seems it lowered the drops remaining.
Now I have 2 cards with 2 remaining out of 8.

All I got was random spam in my chatbox while I was out shopping.
Perhaps it's some glitch that's occurring because they're going to release the Summer Sale cards tomorrow.

Now it seems like the market is down. Maybe there was a bug and they are fixing it. I didn't get any random cards :(

I have just got some cards of Dead Island Definitive Edition.

One Cities Skylines card here just now (not playing anything at the moment).

Judging from the game titles people have mentioned so far, and typical gamer habits, I'm wondering if these card drops are at all related to the games that you chose to play or idle for the Spring event.

[edit] And now (June 15) a Saints Row IV card.

yea i got some cards it says i " Earned due to game play time "
and i only played the game for 3h and already got some cards before
image

7 cards from 5 different games so far, all ones I haven't played in a while.

This has happened to me with different games. Maybe only ~3 times, though.

I got a single card for Goat Simulator... and it was a card I already had

I had 2 darksiders 2 cards show up in my inventory but i only probaly played an hour or 2 of that gamne and awhiloe ago.

Got two Sanctum 2 cards randomly... Not touched that game since 2013.