I understand your concern cause yes it was indeed bizarre for Brett to do so, but we're missing a chunk of info about what happened in the 2 year gap when they were apart which might have been a major event in Brett's life - so much so that it released a deeper plane of reflection within him that was trapped behind the mirror of how he should be leading his life based on the way his father raised him. And that must have been like a big realization moment - the person who liberated him from all that, the time he felt the most free was indeed with Hayden. Which is what brings us to the situation in the current present of season 3. And all this may feel too sudden and drastic cobsidering this is Brett we're talking about but I'm sure there's relevant explanation behind everything. I could be wrong tho, this is all just speculation.
I will say this atleast and you can decide to wait if you like
The latest raws really highlight the shift in Brett and Hayden’s dynamic. With Scott filling in the gaps of the last two years, Hayden finally has the clarity he needs to judge Brett’s past actions. Honestly, it’s refreshing to see the roles reversed. Watching Brett, the one who messed up, fall the hardest and be forced to do the chasing feels like the perfect redemption to his feelings.

Wow what a disappointment final season, after s2 finale i expected different direction