大人的屁股是由多种因素引起的，包括遗传、生活方式、饮食习惯等。以下是一些可能导致大人红屁股的原因：

1. 遗传因素：某些家族中可能存在肥胖、臀部发育不良等遗传基因，这些基因可能会影响到臀部的形态和大小。

2. 生活方式： sedentary lifestyle, poor diet, lack of exercise, excessive weight gain or loss, smoking and alcohol consumption can all contribute to the development of overweight and obesity which can lead to redness and puffiness in the buttocks.

3. Hormonal imbalances: Some women may experience hormonal imbalances during pregnancy or menopause that can lead to a puffy appearance in the buttocks area.

4. Skin conditions: Certain skin conditions such as eczema, psoriasis, or acne can also cause redness and puffiness in the buttocks area.

如果您发现自己有红屁股的情况，建议您咨询医生进行检查和诊断，以确定具体原因并获得适当的治疗。同时，保持健康的生活方式和饮食习惯也非常重要。