How are you heating it? Are you talking about after adding it to the hot or boiling water?? It will foam up some in start with, that is called the hot break, and that is normal.
Hot break. Coagulation of proteins with heat. You can leave it or skim it. Everyone has opinions. When you cool it you will also notice the same thing happening. It's called cold break and will drop out after fermentation. Both are signs of a healthy boil.
LME behaves the same way as an all-grain or DME wort for the most part. During it's process it has been boiled and dehydrated significantly, but there is still potential for break material.