I don't have a solid theory of why, but I know it's happened ever since BGS was a thing, in the live game anyway. I started doing BGS stuff in Feb last year and it's always periodically had the "phase shift" happen every week or so. Only since PP dropped do we have the thursday "drop everything" day. Which reminds me, I need to start gathering evidence to prove to FD that influence is getting dropped on the PP cycle day.
My prediction is for the next few days, up to the PP cycle change, it'll be around 21:30 gmt
The apparent link to the PP cycle day is only a theory and may be confirmation bias, in that it is most noticed when it coincides. There have also been instances where the tick goes missing with no correlation to the PP tick.