You are not the only one that has been mystified by this.
It has nothing to do with you attacking the drawn clan,the opposition sets up what is known as, 'Siege of the Stronghold'.
It costs a lot of resources to start and use.
You may or may not have seen this opperating,or may have done so, but not realised what was happening.
You may have seen the announcements in the chat,if you then go to the location, you will see birds flying round the towers.
The chief complaint about this,apart from ''we didnt attack'',but lost, is that it gives a big advantage to those clans that can use this feature, against those that cannot, or have lower achievements and skills.
They do not, I believe, to even be attacked by real players, as the game sends game troops.
Plarium generates marches that will attack them when doing their siege of the stronghold, so they need to defend against them and your clan at the same time. These marches gives points just like attacking your clan members.
Its a fun mechanic and is what makes cvc interesting, or else there will never be a fight more or less.
You need to gather your clan members and do an "Onslaught" that combine your forces for 1 big attack.