Danny- Often requested feature...
Bunq started as a kind of payment method, but grew out to be a bank. This is one of the things that dates back to that era.
Most bunqers used to use bunq as a secondary bank, so top up from an account at another bank seemed logical.
For those who use bunq as their primary bank, the feature of top ups in its current form makes no sense. They have no other bank to top up from, but still have to do this manually.
You can use the top up messages as a reminder to top up your account, and then top up from one of your other bunq accounts (go to the account you want to top up and hit the Add money-button).