Cameroon is very mad at Liverpool because Joel Matip doesn't want to play in the Africa Cup of Nations.
Matip is a senior-capped Cameroon international, has had disagreements with the country's sport governing body before, when he was playing for Schalke, and has evidently decided he just doesn't want to play for them anymore.
Naturally, the Cameroon FA decided that of course Joel Matip wants to play for Cameroon, who wouldn't, and it's all Liverpool's fault he's not going to the African Cup of Nations.
This, from the Guardian, gets to the heart of the issue:
Liverpool actually thought Matip had retired from international football, but if so the Cameroon coach Hugo Broos did not appear to get the message and named him in his squad. The German-born defender still has no intention with joining up with Cameroon in Gabon but Broos has indicated he may ask Fifa to invoke sanctions. If the ruling body suspects Liverpool of influencing the player’s decision or preventing him from joining up – something the club are adamant is far from the case – they could prevent him playing domestic games for the duration of the tournament.
We'll be eagerly awaiting the conclusion of this one. Liverpool look to have a pretty strong case here, but if on the off chance Cameroon voted for Gianni Infantino in the most recent FIFA extraordinary congress and England voted for someone else, anything can happen.