I don't know seeing how the fallen was the freaking boss you expect shockwave to obey him or maybe fallen tells shockwave to stay on cybertron. I don't know I think of this sometimes
I dont think Shockwave had anything to do with the Fallen, nor should he. Fallen was looking after his own Earth related business and Shockwave will have most likely been doing the same on Cybertron. Also, despite having lots of potential, the Fallen character in the movies was a bit shit and nowhere near what he was in the comics. Especially that awful design. He's best forgotten about to be honest. I think this movie will be disconnecting itself from ROTF as much as possible and rightly so. But you never know. It's not impossible.
Maybe a flashback where Megatron and Shockwave are side-by-side fighters on the Decepticon side, then The Fallen picks Megatron as his true heir instead of Shockwave, causing Shockwave to be a bit envy and hateful towards Megatron [because Megatron denied his leadership]. Also making his treason of Megatron more reasonable. And, it would give him a nice "Scar" [from The Lion King] touch ["Don't turn your back on me, Shockwave!", "Oh no, Megatron, but perhaps YOU shouldn't turn your back on me..."]
I think that Megatron was the only one who dealt directly with the Fallen, at least until he died and SS "took over". Let's just forget about him.