A thing to remember - Player booking can occur at any moment if the right conditions are met. And those conditions vary from the disposition of the individual player, the Squad's formation and tactics (Hard tackling and such), what the opponent has set up for you over at their side, the morale of the player(s), etc. Basically what HoofDaddy said.

Is it random? There is an element of randomness. And that random chance can occur when, as I said, the right situation presents its self. It's part of the game, both Top 11 and real football.

Let's not forget that this is a thing:
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Definitely depends on individual players as well I'd say. Back in July 2016 I formed a new squad of players who didn't get more than a couple of yellow cards for then next several seasons. Right now though, hah already three red bookings from the two new guys in the squad. (I'm thinking of sending them off to military school to learn some discipline lol)

Anyway, it's part of the game as I said.