One of the assumptions behind the use of radioactive dating is the idea that the rate of decay of radioactive elements is essentially constant over long periods of time. There are a number of problems with this assumption. One of the problems is identified in the linked article (link to article). As identified in the article, the apparent rate of decay seems to have a connection to the Earth's relationship to the Sun and perhaps the Earth's relationship to a slowly spinning core of the Sun. I'm sure that some folks will be quick to point out that the variation that has been noticed is periodic and is small. This seems to be true. Nevertheless, the point is that there is already some observed variation, and (so far) the cause of this variation is not known. There is speculation that it is caused by neutrinos, but bluntly put the scientists who are studying this are just guessing what the cause might be.
Furthermore, while scientists can observe these short term trends, scientists don't have the data to see whether there are long term (even on the order of 100 years) trends. They don't really know that radioactive decay is constant - that is assumed based on an apparent constancy at the present time.
And that assumption is open to very reasonable doubt.