Changing gravity could certainly be explained by super dense mineral clusters. Once you get below the mantel the Earth is essentially a liquid… it isn’t so beyond belief to imagine a world where mantel temperatures are higher and everything below the thin outer crust is fluid… if we then imagine pockets of super dense material with weird magnetic properties it’d be possible for large clumps of that to float through the mantel and cause interesting variations in gravity. Gravity follows the inverse square law so a super density fluid traveling through the upper mantel would potentially cause some really odd effects.