Why is GoGo mounting the other boys

OK....I've been meaning to post this question for quite some time...here's the situation....I have 4 male neutered boys…

Every now and then I will hear a strange MEOW coming from Spot or Peanut and when I check on him...GoGo has him pinned—meaning, he's biting the back of Spot or Peanut's neck and he appears to be 'mounting' him????? He doesn't dare do that to Rocko, because little Rocko would open up a can of WhoopA$$ for sure…

What the heck is this behavior?

Thanks All!
Kim

I asked the same question over in the behavior forum, and some suggested it may be a alpha cat characteristic - here it is below:

http://cats.about.com/gi/dynamic/off...h%2Findex.html

I'm moving this over to Behavior.....and reading the replies. Squirt went through a period where he did this to Joey every time the poor thing stood still!