Why does my pizza dough (and other doughs) go sticky on freezing and defrosting?
There are several effects which I think are at work here:

starch hydrates over time. Sure, within an initial kneading, it already hydrates quite a bit, but if you leave it longer, it continues changing. (That's also why it is recommended to rest crepe batter). So the starch which was somewhat-dry is now completely wet, making the dough as a whole stick. If you had patches of completely dry flour on the outside left from shaping, they are now also completely wettened, giving you more initial sticking at your first touch of the dough, which probably contributes to the sticky impression even more.

24 hours is quite long for an already-risen dough, even though the temperature is very low (starts from frozen). I wouldn't be surprised if the overleavening process starts already in that timeframe, and overleavened dough is sticky.

the gluten mesh is susceptible to physical disruption. I mostly notice this with home-ground wholemeal flour, the bran basically cuts it too much for proper rising. I guess that the formation and expansion of ice crystals also has a bit of an effect on your gluten, even though it's milder because of the lack of kneading while the crystals are present.


So I think this is to be expected, and not exactly worrisome. If you want to reduce the effect, you can freeze the dough in discs instead of a cuboid or cylindrical container. You will then have shorter defrosting times, and your dough won't overleaven on the outside while the center is still defrosting.