It was a deliberate trip, let's not pretend he just slightly mistimed a genuine attempt to play the ball, there wasn't a split second where reaching the ball was in mind for Herrera
As you say, deliberate trip is a fairly obvious yellow, if it had been just a mistimed tackle then exactly maybe not a yellow, though you have to factor in the first yellow was a nasty foul and he mouthed off heavily to the ref. A lot of refs would give a yellow for that shoulder barge then give the player a talking to and a warning for mouthing off about the decision then send them off for the next thing they do.
Also if you just continue to foul one player, but rotate through the team then 11 players might get 1-2 not great tackles in on a single player before anyone is even booked. If the TEAM tactic is to kick seven shades of **** out of a single player as a tactic, that tactic deserves punishing. THe ref gave the captain a warning that if they continue to target him the next players making blatant fouls get booked.
In either case the second yellow is easily explained and entirely justified.
He wasn't just late, he was beaten all ends up so he stuck a leg out to trip him. Lately there seems to be this attitude that if you trip a player but don't like two foot them and risk breaking their leg then it's not a card. Conveniently forgetting that you should get a 3 game ban for violent or reckless conduct like a two footed challenge, but a one game ban for a professional foul/2 yellow cards, they aren't at all equal. Same thing has been happening when it comes to kicks, a few players have kicked out recently but fans saying it shouldn't be a red because the kick wasn't very hard or didn't really make contact which has nothing to do with it being a red.
A soft yellow is when someone makes a genuine tackle and it's marginally mistimed or just missed the ball, a soft yellow is not when you're beaten and you just trip a player.