That's a good hypothesis, though in that case I would assume the surge in interest in Mint would be accompanied by at least steady interest in Ubuntu instead of a drop. Also, the interest in Mint has been rising for quite some time and interest in Ubuntu has been decreasing steadily. This could just be that by now everyone who's interested in Linux knows all they want to know about Ubuntu, but interest in alternatives is starting to increase.
My premise has less to do with the particulars of the exact numbers of Ubuntu vs. Mint users, and more to do with the fact that there's not really a go-to distro for new users anymore. Ubuntu, Mint and Peppermint are all contenders for this role, and openSUSE and Fedora are still options as well. I think this lends to an uncertainty about which to choose and which will have continued support. The problem is compounded by the fact that a lot of applications are being accessed from browsers and/or mobile devices, so that even as Ubuntu could be gaining power users its pushing power users away in an attempt to gain mainstream users that they may never get.