I think it is googles javascript code.
It only happened to me if I had many zero-k pages opened.
I think about 8 or 10 ^^-
Since I don't refresh the PW pages so often, the trade center changes only once a week, and I don't have to engage enemy fleets in the last moment, I don't notice this issue more than once or twice a week.
I think It is/was the tabbing script - if you refresh your page with a link like
http://zero-k.info/PlanetWars/Planet/12353#TRADE-TAB ( It is important to have a #ANCHOR in your link to cause this issue more likely )