Basically yes, it's very hard to predict when it will happen except that it's usually a few months after the last free trial.
I don't think its literally random, someone at ZOS (or a team of people) decides when to have a free trial so I assume there is some logic to it, but it's not on a fixed schedule and we don't know how they decide when to do it, so it's not possible to accurately predict when it will happen.
I've just had a look through the announcements on the main website and, assuming I found all of them, previous dates have been:
May 2nd - 6th 2019
August 8th - 13th 2019
January 17th - 23rd 2020
April 20th - 27th 2020
October 12th - 19th 2020
January 19th - 26th 2021
June 14th - 21st 2021
So they're roughly between 3 and 6 months apart, and the last one was in June, so there will probably be another one before the end of the year, January at the latest, but I don't think I can be more precise than that.