I think my guinea pig is hurting his brother

[Elle's Pigs]

I have 2 guinea pigs and they are brothers, they're about a year and a half old. One of them, Tomato, is quite a bit bigger than the other, Kelpie, and when I first got them Tomato was definitely the dominant one. But now, Kelpie is showing strange behaviours. He often tries to climb on Tomato, it's not mounting, rather he will try to stand on Tomato. He doesn't exactly mount Tomato but he does bite and stick his face in Tomato's rear. I noticed him nibbling at Tomato's ears and I thought he was just grooming him, but Tomato gave a shriek of pain and his ear was bleeding. Whenever Kelpie does these things to Tomato I separate them for a little while, even earlier today I had to pull Kelpie off from Tomato's back. They live in my room and Kelpie never does this when I'm studying or reading or focused on something else, but whenever I get up/get in bed or come in and out, Kelpie starts to harrass Tomato like this. In other words, he only does these things when he knows I can see it. My biggest question is, what is Kelpie doing, and what caused the behaviour? What can I do to change this?

 

[Piggies&buns]

Welcome to the forum

Are you certain it’s not mounting?
How big is their cage?
Are their weights stable at each weekly weight check?
Are their multiple resources in the cage
(Ie two hay piles, more than two hides each with two doors)?

If you believe their bond may be failing, then the best thing is to carry out a temporary separation for a few days to let them calm down.
After a few days apart you can reintroduce them in totally neutral territory (never reintroduce in the cage) and see how things go.
If they no longer want to be together it usually becomes quite obvious quite quickly once they are in neutral territory.
If their bond has failed then they will need to live in separate cages permanently.

If all goes well after several hours when they are in neutral territory, then you can clean out their main cage and move them back to it together.

The issue becomes if they are actually hurting each other and having actual fights. If that is the case, then a bond has already failed and you separate them immediately and permanently with no chance of reintroduction.

Bonds In Trouble

Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?