Ticklishness can vary from person to person and can also depend on the sensitivity of certain areas of the body. Your mom likely had previous experiences that made her aware of her ticklishness in many areas, but the neck (or "pescoco," which is Portuguese for neck) might not have been one of those areas she expected to be ticklish.
It's also common for people to have surprising reactions to stimuli in areas they don't often think about. The