It is normal that they would chew wooden sticks or pretty much anything wooden. Remember they are still wild animals and in the wild, they chew thru tree limbs to get to the sap. That part of it could just be natural instinct.
What was the tunnel made of?
The first thing I would do is take them to the vet for a wellness exam and fecal float and smear. Rule out any medical issues.
Then add more toys and items to stimulate them while they are in their cage. Change them out every couple of days.
And we need to know exactly what you are feeding them to see if their die is deficient.