No, I have never heard/read any detailed explanation about this phenomenon and I have tried searching the web in the past because I, too, have been curious about it. Typically you'll just see the preparedness advice to keep the plastic containers up off the bare concrete floor, like putting your 50-gallon water drum on a wooden pallet or keeping your smaller containers on a shelf.

Any civil engineers or construction types out there? Maybe they'd know.

Edit: I was just Googling a bit and ran across this webpage. Obviously, this webpage is a sales pitch for a particular product (which I'm not endorsing in any way), but it does provide some interesting insight into why keep your water off the concrete floor. Apparently, even if you never spill any water on your dry concrete floor, there could still be water present as water vapor, which can leach chemicals out of the concrete onto the surface. Again, not intended as a concrete explanation (excuse the pun) but does shed some possible light on the subject.

Oh, and I did read that concrete used for water storage/transport purposes needs to be certified concrete, so I assume that a concrete water reservoir is not necessarily using the same concrete as what is used in my garage floor.