Why do parents sometimes act like children?

My mother and father have been divorced for 18 years now. Dad is remarried, Mom is single. They've never had a particularly cordial relationship, and even had me crying the night before I got married when Mom refused to move her car slightly in the parking lot of the little hotel we were all staying in so that Dad could also park his. There was plenty of room for both.
So Dad and his wife were visiting us for the first part of the week and were condo-hunting in town. They found a place they liked and bid on it. We looked it up online and were around when they were discussing one of the offers with their real estate agent, so we knew how much they paid, which we wouldn't discuss with anyone else because it's a private matter of theirs.
Fast forward to today, when Mom is visiting and starts asking about what they were doing in town, and did they find something. How big? We told her. Then she had the nerve to ask how much they paid for it. When we declined to tell her (first trying to say we didn't really know, then when she asked if we knew and just didn't want to tell her, saying no we didn't want to tell), she got into a big snit, told me that I always make conversations about my father so difficult, and stormed off.
We both feel she was totally inappropriate asking about how much he paid for it. She always likes to go on about how he's rolling in it (we don't know if he really is or not), so we also don't need to add fuel to that fire.
And so now she's been hiding out somewhere (maybe in the guest room in the basement?) for over 30min, and didn't even respond when I shouted down that her granddaughter was done eating and was ready to play.
ETA: she came up ages later, walked into the family room looking furious, and sat down to glare at me and tell me how I always put her in a bad position when it comes to my father, by refusing to talk about him and making her ask questions. How I don't need to make it always so awkward to talk about him, just because he left and married someone else and I'm mad at him. At this point, 18 years later, I couldn't care less, apart from the annoyance it causes with my mother. I told her I don't mind talking about him and what he's doing, but it's inappropriate to discuss his finances with anyone but him. Ugh. So complicated. Found out she's still mad at him 18 years later for not apologizing to her for leaving and marrying someone else.