Not shady at all - inventory is limited and not guaranteed.
I would bet that the inventory looked something like this Y7 Q7 M7 L7 T1 as an example. As soon as OP hit book the booking engine went out to grab the one seat left in T class. The system, as noted, will not give it to two people who are asking as there is only ONE seat in that bucket.
The same thing would happen if you tried to book one seat and saw the fare based on T class and then went to book two seats. You cannot combine inventory from different classes on the same flight segment for two passengers. As such, in this case, Delta (and all airlines) would display the next higher fare where two seats are avail to sell, L.
Nothing shady. They had one seat, OP wanted two seats.