Hello all. Brief intro, I live with 5 dogs and a husband

The two youngest ones are Polly, female Am. Staff. Terrier 1.2 years old, and April, female Am. Staff. Terrier, 1 year old. (all my dogs are altered)
April, the youngest came to us two months ago. She is a shelter dog with unknown past. All the others came to us as tiny rescue puppies.
Back to April: sweet, reserved, not overly active, loves to snuggle, showed mild food aggression on day one, but we nipped that in the bud, it's not an issue. She goes nuts when the doorbell rings and attacks the door. But we are working on desensitizing her to the door bell by keeping her on the leash and having neighbors ring the door bell, and guide her through the response.
Other than that, she is a sweet, sweet dog.
Polly is the other young pibble. Polly is fun loving, high strung, nosy, non-confrontational, loving, always having to be in the middle of things, moderately high energy. But all that really does not matter in this case. The two of them get along famously! We were so happy to have found a great match.
Last night, out of a deep, deep sleep, no warning, no growling, April attacked Polly, who was getting ready to curl up next to April as they always do.
I pulled April off of Polly, who wet herself and was terrified (non-responsive), and April went right back after Polly. I separated them again, locked April in the bathroom for a while, checked for damage (none), and reintegrated April into the living room.
April was very aloof but friendly, did not play, was not threatening - like in a trance, like the first day she came here ... very uncommonly unattached.
I have lived with dogs for 25 years. I volunteer in rescue for the past 10 years and thought I was pretty well versed with dogs ... but I have never seen anything like this. I mean out of solid deep sleep and went straight for her buddy, was on top of her, and just did not have time to sink her teeth into anything - I think, because I was too quick.
Does anyone have any educated guesses? PTSD? Seizure? Indigestion? or crazy dog?