I have been watching the seats. There two sections of seats barring the exit and super big seats in front. There are the regular slightly cheaper seats in the back, now rows 15-19 and the front like rows 6 to 11. Left out rows are not showing available.

So for 10pm flight checked at 9am and 11am. Row 15 went completely before 16-19,, I doubt in the two hours people bought all 6 of those seats though its possible. Still not sure if they do aisle or window first.

Price also fluctuated in 2 hours. The front and exit went up from $18 and $25 respectively to $20 and $27 while the back section went down from $25 to $16 (why they were higher than others makes no sense)

So my thoughts , different than BE on AA since everyone is equal (no basic vs regular on Spirit), is that they will fill the back section (16-19) from front to back and then fill the front section from front to back (6-11)