Lakes are low on saturated oxygen right now but it generally shows up on certain parts of the lake, not all of it.
As far as "turnover"...it's not going to be turnover that you are seeing. Most north Texas lakes don't experience true turnover. Our lakes mix.
Turnover happens when we have a very harsh cool down for an extended period of time "when" the lake is still warm. So, if it happens it will happen generally in mid to late October. DFW's earliest recorded freeze is OCT 22.
On the contrary, our lakes mix pretty well. Mixing is when cooler surface temperates begin sinking into the lower part of the water column at a slow pace...mixing the dead and decayed oxygen depleated water with cooler oxegen rich water.
Even during the mixing process you will smell some funky water in certain areas of the lake with foam, etc.
I think you were seeing a small thermocline on ur sounder...