I can't answer the "why" but Ted Chen has been talking about it on the 7 AM and 6 PM Saturday (which was back to a full hour at 6 tonight. No "Malibu Polaties" infomercial during the second half hour).
He said it's a new show called "Sunday Morning" and will be more informal. No coats or ties...more relaxed. I'm not going to guess why they are doing it but I think it's dumb when KABC runs up to 4 hours of local news on Sunday morning.