Northrop Grumman Manufacturing Engineer interview questions

HR Business Partner reached out to schedule an interview and set up the following week. Interview with hiring manager went great! Received an offer over the phone from HR (Wednesday afternoon). Tried to negotiate salary a bit, with no success. I asked if I could have the night to decide and discuss with family; HR agreed. Called the next morning to accept the offer. HR said "great, I'll let the manager know." Got a call 4 hours later saying the manager wanted to "continue interviewing stronger candidates". Very unprofessional experience overall. Not sure why NG is extending offers to "weak" candidates.

I was contacted via email and we set a date for when the hiring manager could call me. He was the engineering manager for the entire division, and he told me there would only be one interview.