Just brainstorming a semi-plausible explanations here. What if the variation is due to massive portals/wormholes to other planets? If you're standing near one that goes to a place with much higher gravity when it opens up, it could cause you to be pulled toward it, or increase gravity around that area. If these portals are kept secret, the gravity fluctuations as they open and close might appear to be as random as weather patterns.
Could be an interesting plot point too, if your story includes races that have secretly come through these portals. Their existence could be discovered by triangulating the gravity changes during an event. Lots of interesting possibilities.
Peep Show is my human litmus test. Seeing how people react to that show can tell you a lot about them.