Messages transferred automatically from "active message" to "dead letter queue"

Hi,

I would like to understand the behaviour of service bus regarding the dead letter queue.

We are using service bus premium.

I send message to a queue in order to reach the queue size limit.

When the queue is full, let's say I have 20k messages in the "active message count" and 0% free space. 

When the queue is full, I stop sending new messages.

The next morning, I can see that all my 20k messages are now in the dead letter queue and my queue is still at 0 % free space.

If I read the message in the dead letter queue, I can notice that the reason is : 'MaxDeliveryCountExceeded': 'Message could not be consumed after 10 delivery attempts.'

Can anyone explains this behavior ? 

Thanks