There are multiple theories as to why the 2H levels on the planet have been rising.
One is that nuclear power plants synthesize high 2H water to cool their reactors. When there are nuclear power plant accidents, millions of gallons of this synthesized high 2H water is released into the ocean, which eventually ends up in our air, food, and water.
After hundreds of accidents over the past 50 years, it's possible that the 2H levels on the planet have risen significantly.