During the last Clan War, my clan was level 17. Then when the war had ended, during the 48hour period where the rewards are handed out, we got shot up to clan level 19. We were around 20k xp in to level 19 and completely jumped level 18, meaning we gained over 100k clan xp. (We came 2nd place in that clan war)
I noticed many other clans getting big xp bonuses and others not gaining so much at all.
Trey_Beachhead confirmed to me on Twitter that yes, xp bonuses are given after clan wars. It would be interesting to know what determines how much is rewarded and what can be done to maximise the amount given.
I've been told that it is just from playing with clan members during clan wars, but that is extremely vague info. So, you're welcome to discuss here anything you may know or find relating to this! Would be interesting to see what my clan are able to do in order to get the most out of this xp bonus given from clan wars.
Because I didn't pay attention the first time we got a big jump in clan Xp I did paid attention this past clan war and when we played together we got our normal clan Xp and after the clan war we received another nice chunk of clan Xp . So I would say this is more for winning or doing well in clan wars since we were already awarded our clan Xp for playing together. I've looked and asked just haven't seen anything official as to how much or what the criteria is For the bonus clan Xp.