After 4-wheel alignment, handling is very twitchy.

[Retroman1969]

I've been meaning to post this question, but keep forgetting until now.
A few weeks ago I put on new rear tires and had a 4 wheel alignment done along with having much of the negative camber taken out of the rears for a more neutral tire-saving setting.
Ever since, the handling of the car has gone from smooth and linear to twitchy and unstable.
It's hard to describe, but when steering into curves or making quick maneuvers, instead of the rear of the car following the front smoothly, pushing out a bit, it suddenly "tucks in" and "grabs" throwing the balance off.
I was alerted to how bad it was when I had to make an emergency maneuver on the turnpike yesterday.
Some idiot in an SUV comes charging on from the on ramp. I'm in the middle lane and the blind ass keeps charging across the lanes right into the side of my car, so I have to swerve into the next lane to avoid a collision. It's something I've had to do before as I'm sure many of you have at one time or another. Sucks, but no big deal. You quickly move the car into the next lane. But with the new handling quirk, the rear tucks and grabs, and suddenly throws my car sideways at 65 mph! I try to keep calm and gently start counter-steering, but the car just flips the other way. I have to keep correcting back and forth until the chassis finally settles and corrects itself.
I'm not a panicky driver. The avoidance and corrections I made were very minor and done with a cool head.
I'm now very aware something is wrong here creating very unstable handling.
Any ideas why a simple alignment could so dramatically change the characteristics of the car? Or could it be the tires? (I sort of doubt it, but...)