[yuchi]

Since those are percentages it could simply mean that a great amount of Java positions opened up in that period, stealing percent points from other categories. This explains the symmetry, but not the causes.

May it be a huge amount of positions have been incorrectly labeled for Java instead of JavaScript (you know… Ham / Hamster) and then corrected a month later?

[WillPostForFood]

Or the absolute number of Java listings is pretty much constant (permanent, big corps that just leave listings online all the time), and as smaller web companies add JS listings, and remove JS listings, that fluctuation makes the Java percentage move in inversion to the more volatile JS listings.