I think thats too obvious. What is GRRM not saying directly about the brotherhood?? Thoras is depressed, or worse....disenfranchised. Lem is a different man, as well. Tom as lively and optimistic, and is now not in the group.
If its not the Hound saving her, then its Thoras. Brienne oozes chivalry, determination, and virtue. The qualities Thoras saw in Beric. And as for Sandor, he would have seen it at the island. He dispises knights, seeing them as two-faced and self-centerd. Brienne is anti-knight. She is everything they should be, but nothing like they really are. Sandor and Thoras may very well fall enamored with her iconic personae.
But even if you're right about Tom, it doesn't diminish Thoras's response to Brienne. He loathes the new Brotherhood.