Hi and welcome
Yawning is a defensive measure usually used by piggies that are generally somewhat out of their depth, afraid and on the losing side of the dominance sort-out. It translates as 'Keep away from me and don't push me. I might defence bite.' with the latter part not invoked if the piggy has got enough space to run away when challenged.
Piggies have to re-establish their hierarchical group in any new territory. Yours have obviously not settled in fully yet. Keep in mind that guinea pigs are prey animals with prey animal instincts. You also don't know how they have been treated in their previous home.
Here is more information on social interactive guinea pig behaviours:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Dominance Behaviours In Guinea Pigs
Arrival in a home from the perspective of pet shop guinea pigs
How Do I Settle Shy New Guinea Pigs?
Understanding Prey Animal Instincts, Guinea Pig Whispering And Cuddling Tips
How To Pick Up And Weigh Your Guinea Pig Safely
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