I'd say that lack of knowledge of the rules does play a role.
For example, people who think the black card is for any cynical foul--as opposed to a clearly defined set of offences-- will feel aggrieved when one isn't given for a foul on their county player, which, while cynical, isn't actually a black card.
Or people who don't know that a red-carded player can be reintroduced during extra time (which is most people, as far as I can tell), will feel that they are being cheated out of it once an opposing red-carded player is reintroduced in extra time.
These perceived abuses add to a general feeling that the world is out to get you, and that adds to the venting on social media.
Or so it seems to me, at any rate.