My guess is they didn't think they could re-sign Kahun.
That said, why would you trade him now? I like Dominik Simon, but I'd much rather have Kahun at 2-3 years at $2.75M to $3M a season over Simon for 2-3 years at $2.25M to $2.5M, where is about where they're both going to be. So why on earth would you trade an impending RFA, for an impending UFA who you aren't going to be able to afford anyway?
I don't like the trade. I don't care if Sheary had chemistry with Crosby in 2017, he falls down too much. He's less productive than Kahun and he brings a lot less to the table. Evan Rodrigues seems like a depth throw-in, not sure what to make of him. I liked the Marleau trade, but this one is really bad. I don't think it kills the season, but it makes you wonder if Mike Sullivan didn't like Kahun or just liked Sheary more (Which is odd considering he was in Sullivan's doghouse) in 2017-2018. The funny thing is, we trade Maatta for Conor Sheary and Evan Rodrigues, Sheary was traded to make room for Jack Johnson. So we traded Olli Maatta for Jack Johnson.