I think what the Bruins were going for was they were going to leave 2 of the 13 forward roster spots for two of the young guys. They figured out of Frederic, JFK, Studnicka, Bjork, Donato, and Cehlarik, two of them might make it, and they were hoping one would be a winger and one would be a center. 11 of the 13 spots were accounted for regardless of training camp performance. Noel Acciari could of pee'd in Cassidy's morning coffee and he was still making this team no matter what, as was the rest of the 4th liners. This organization has it's mind made up what type of 4th line they desire, to the point where they go out of their way to make it so.
This was also impacted by both one-way contracts and waiver eligibility. If you look at their line-ups from pre-season this seems pretty clear the initial plan.
Marchand - Bergeron - Pastrnak
Debrusk - Krejci - Heinen or Young Winger
Heinen or Young Winger - Young Center - Backes
4th line - combination of Nordstrom - Kuraly - Wagner - Acciari
So a young guy had really zero chance of a 4th line spot.
The problem began with no young winger stepping up and really grabbing that spot alongside Krejci and Debrusk. Heinen was tried there but it really never worked.
Then no young C looked ready coming out of camp. JFK was underwhelming in pre-season/camp. Frederic looked a bit overmatched against pre-season line-ups with mostly NHLers. Studnicka was always up against it with him having to go back to the OHL if he wasn't in Boston.
The hair in the soup was Backes didn't and doesn't look like a Top 9 player anymore, and really is just one more veteran 4th liner to add to a roster that already was carrying four 4th line players.
Then Heinen wasn't producing so it amplified the issue with Backes.
So it all snowballed into a perfect storm of issues putting together the 2nd and 3rd lines.
@LSCII hit the nail on the head when he said these recent moves are all about Backes. The experiment IMO with Backes w/ Debrusk and Krejci I think changed their thoughts on what to do. Backes looked like a fish out of water with those two. Backes himself talked about predictability and playing with two guys who play "simpler" games like Acciari and Kuraly should make it easier on him. You could see with Debrusk/Krejci he wasn't sure where to go and did a lot of puck watching.
So I think they've decided that if Backes is in the line-up they need to play him with somewhat similar players. And if they aren't using him on the 4th line, they need similar players to Backes on the 3rd line.
Hence the recall of Frederic.
The fallacy of this method IMO is having such a pre-disposition to what they want each line to look like, it really hand-cuffs them in terms of roster and line-up decisions. Personally I don't think that works but that's what they do.