sudden onset of non-stop mounting

Question
Hi. I have two male labs that are not neutered.  One is 7, the other is 8.  They would occasionally, I'd even say rarely mount when excited by company over or something but for four days now my older male is suddenly mounting my younger male lab non-stop all day long unless I separate them. The older male has alot of tumors that the vet has labeled "fatty tumors"  Some are very large and lots of small ones. I don't know if this has anything to do with this, just wanted to mention it.  What is wrong with him that after 8 years of normalcy around my other lab, he is suddenly doing this constantly?  Thank you for any insight you can give me.

Answer
It could be that your older dog has either decided that this is feel good behavior, or he is feeling vulnerable and trying to assert social dominance over the younger dog.  Neutering him may help, but I doubt that the tumors have anything to do with this.