Random new address created

I seems that my wallet moved some funds into a new random transparent address that I did not create.
I am mining at zmine.io, and have always used the same address.
After receiving 3 payments and having made one payment I noticed my balance was not in the one address I had been using but in two different addresses. It seemed strange that this new address got created, is this normal?
Also, is there a way to send the balance of several addresses as payment. In other words, if your balance is distributed in several addresses must we use a distinct z_sendmany command for each? I wonder because the wallet makes me pay .0001 per transaction, and if I have many micro-payments in many addresses it would become expensive to make payments.

it's normal

This comes from the ZCash implementation of z_sendmany.

The GUI wallet currently has the limitation that sending from/to many addresses at the same time is not possible. This will be implemented in future versions.