Male guinea pig humping other Male guinea pig face?..

[Myana :D]

So I have two guinea pigs
one is 5 months and the other is a year and 8 months
the older one keeps humping his face and idk why :/

 

[Piggies&buns]

This is a very normal dominance behaviour and something piggies will always do.

The guides below explain further

A Comprehensive Guide to Guinea Pig Boars
Dominance Behaviours In Guinea Pigs
A - Z of Guinea Pig Behaviours