Fowler had a 4 team trade list, which is about as close as you can get to a NTC. Of course, they could have threatened to waive him but then they wouldn't get an asset, and he actually might have cleared at his full cap.
Getting a 2nd back for a 4th is pretty good in the scenario.
They basically gave up two 4ths for a 2nd and made what in their situation is close to a lateral move with Fowler swapped for Trouba. Not to mention, they will probably get a 2nd or 3rd round pick for Trouba next year at the deadline, or even in the summer if they retain or take back a contract.
They are a budget team, so that's why I think these two things are linked. Trouba makes 6M in real money, and they shed about 4.6M with Fowler and 1.1M in 1-way money with Vaakanainen, who seems like he played his way off their team and was destined for waivers. So if he wasn't going to take up a roster spot, it's really close to real money in/real money out.
It's a nice series of moves for a budget team. Trouba for Fowler is a lateral move being close to cash in/cash out (not saying they play the same way, but they both will eat up 20+ minutes for Anaheim), but to make that move they got two upgrade two 4th round picks to a 2nd round pick, and they got the player with less trade protection, which may allow them to accrue more future picks from these trades.