Brian Wheeler wrote in this forum post about the upcoming PVP population cap testing. The cap will be increased in each server (PC and console) for a day between December 6 until December 12. During this time, double AP is granted in the affected Cyrodiil campaigns. 

No update has been given so far on which days are selected for servers other than PC EU. 

While it’s nice to recieve some attention for PVP, there are a few sidenotes to make. The post does not mention why this is happening. Presumably it is related to the new servers installed earlier this year, which vastly increased performance. However, it is already noticeable that slowly these new servers are already experiencing some performance issues again, even with the current population cap. 

We also do not know what the population caps are exactly. 300v300v300 is one of the rumored numbers, as well as more and less. This help page mentions 600v600v600, definitely an unlikely number for the rather low amount of action we see in cyrodiil nowadays.

More details and opinions can be found in this video.