My personal thinking is they realized they had a core problem with this team's defense and they needed to get a lot of fresh heads into the mix.
Sure they needed and still need to upgrade several defensive positions, but they are getting rid of depth and not all of it can be explained away as switching to a new defensive system.
I think this is a purge of the old "bad defense" mindsets and replacing them with players from teams who had good or solid defenses or at least not bad defenses.
As for tipping their hand at the draft, only fans think that's some type of secret.
Now, the first round? Sure. A team doesn't want to tip their hand on the positions they'll be after in the first round, though if you're picking in the top 5 and you don't have a franchise quarterback, most people know that's what you're after.
Beyond that though, teams tip their hand long before the draft because the put word out that they're looking for linebackers, cornerbacks, etc. and all teams know what players you signed or traded for to fill those positions.
Teams may know the Cowboys need linebackers, cornerbacks, edge rusher, tackles, etc. but you could create that kind of list for most teams.
What teams don't know is which ones the Cowboys will prioritize in the first round.