Very possibly, yes.
Let's say there's seven or eight different potential adventure sites, most if not all of them seemingly unrelated to each other, the DM has dropped in as hooks. The players send their PCs out to one of them and wipe it out, then another, then another - doesn't matter which sequence - without ever noticing there actually is a loose connection between all of them.
Maybe, for a dumb example, the sites are all controlled by, or pay protection money to, the Cartel of BBEGs; and so it doesn't matter whether the PCs knock off the Goblin raiders or the spooks in the castle or the Evil Wizard in her tower, sooner or later they're going to attract the attention of the CoBBEGs who will then mount a response of some sort. Meanwhile, by sheer luck the PCs might decide to jump on a ship and sail across the sea*, leaving the CoBBEGs chasing shadows.
So yes it's a sandbox in that the players have full choice over what their PCs do in the setting; but there's still stuff going on behind the scenes. And if they somehow do realize the connection between the sites and stumble on to the existence of the CoBBEGs then that takes things in a whole new direction - maybe.
I've done just this on numerous occasions - embedded 4-to-6 adventure APs into a larger more sandboxy campaign. In my current campaign I can quickly think of five such occasions, and I might be missing one.