Sure, you have to ignore the decline in violence (simply looking at absolute numbers would help in a few cases). But there could still be periodic patterns against the general backdrop of decline. However, there is little evidence for that.
Now all kinds of things could go wrong and lead to rising violence levels in the next years (global financial meltdown followed by a drop in international trade, countries trying to collect on their debts by military force etc., which would eventually have an effect on personal violence as well), but if these things happen exactly in 2020 (and not earlier or later, or not at all), it will be coincidence, and not related to some recurring societal patterns. Our world is already very, very different from the 1970, and no matter what happens, there are so many things which are quite irreversible, at least in the course of a decade or two (the global rise in English literacy, for instance).