Why do some people’s skin sag after Botox masseter muscle? How can I avoid it?

The sagging you are describing can potentially happen after treating a very hypertrophic masster muscle. When the muscle is bulky/ thick it acts to hold the lower face "back" to some degree, almost like adding filler to this area does.  When you relax and shrink the muscle the skin in the area may seem more loose.  We inject this area quite frequently and have not heard this complaint from our patients. 

Thank you for sharing your question.  Although nothing replaces an in-person examination, it would be unusual to have masseter atrophy from Botox lead to skin sagging.  Hope this helps.