I'm noticing it, too, and it's not returning to your kingdom that is triggering it. I had it happen to me when I attempted to vault some gold, so it appears the trigger is when the game refreshes its connection to the server. Also, take a look at your gold. That also is getting a boost.
[added]
It could be GREE making up the the deduction of gold and XP that many people posted about in threads like this:
http://www.funzio.com/forum/showthre...gold-and-or-xp
This is a screenshot right after killing some mobs (it's high because of the bonus gold from whatever is happening right now.)
Then, when I went to donate it to my guild, I noticed I had way more and took another screenshot:
This gave me a bonus of 420,118 gold and also about 18,000 xp.
So, all you campers who whine about gaining XP, should probably just put the game down for a few days. We'll let you know when it's safe to come back.
Also, don't bother trying to find me. I already donated the bonus gold to my guild, so you will get nothing.