According to this story in Politico, the White House worked with senior staffers on the House Judiciary Committee to draft the order. But those staffers, who work ultimately for Committee Chairman Bob Goodlatte (R-VA), didn't tell their bosses. In other words, they secretly collaborated with White House staff without informing the members of Congress they work for. Indeed, the administration went so far as to have them sign non-disclosure agreements swearing them to secrecy!
This is quite simply unheard of.
To be clear, the executive works with Congress all the time to craft legislation. That's the President working with members of Congress, though much of the actual work is delegated to staff. All normal. It's congressional staff working for the executive without telling the members of Congress they work for which is the big deal.
There are two levels on which to understand this. One is simple workplace common sense. If your employee or subordinate does something behind your back that ends up embarrassing you or catching you off guard, that is a major breach. If you agree to do work for someone else and promise that someone else not to tell your boss, your boss is not going to be happy. Now, you may say there are plenty of more important things going on right now than a members of Congress getting insulted or having his feelings hurt. And you'd be right. But if there's one thing members of Congress excel at it's guarding their institutional and official privileges. How does Goodlatte trust these staffers again knowing they went behind his back?
I guarantee you: career staffers who are hearing about this are gobsmacked. It's unheard of.
The second level is more complicated. I'm not sure this rises to the level of a formal separation of powers issue. But the idea of the White House coopting congressional staff behind the backs of members of Congress certainly runs roughshod over the overarching concept of two coequal and separate branches of government.
To be clear on one point, Goodlatte isn't some moderate or softy. He's an immigration hardliner. It's unlikely he would have disagreed with at least the broad outlines of the executive order, though perhaps he might have disagreed with some of the particulars.
There's so much going on right now that I don't know how much attention or immediate effect this will have. Perhaps the White House will promise to act differently and smooth over any ruffled feathers. But these are the kinds of actions that sour relations between the White House and Congress, even of the same party. Maybe it shouldn't be the biggest deal, compared to some of the other things that are happening. But I think it will end up being a bigger deal than people likely realize.