Warning Behaviour?

I have two male chinchillas (10 months and 14 months) neither are neutered and they are a bonded pair (rescued). Recently the 10 month old has started trying to mount the 14 month old (who isnt all too pleased about it, as you can imagine). It started as a little thing but has progressively gotten more feisty. Is this just adolescent behaviour, trying to assert dominance or could it be an early warning sign that they may start fighting?

This mainly happens during playtime- in their cage...

Warning Behaviour?