Why older chihuahuas walk with butt sideways?

Dementia. If it's an old dog or if it's been poisoned, some sort of mental issue could cause a dog to walk sideways. Pain from some sort of internal issue like a bladder infection or trauma could also.

By the way, something I learned recently that the majority of dogs over the age of ten experience dementia. Age, size, health all factor in on when it starts, but most do. There are medicines to help slow age related brain damage - so as soon as you see this in your old dog get them to the vet ASAP to get your fury friend on meds.