I've read a couple of different things while browsing the forum in hopes of finding a clear answer.
The first is that a rise in pH comes from increased aeration of the pool water, thus gassing of carbon dioxide.
The second is that a byproduct of electrolysis (for NaCl water) is Sodium Hydroxide, which is a base.
I don't know if the NaOH is significant enough to change the pH, but it seems possible.
Or maybe it is a little of both?