It's an easy one to miss, but by default, when creating Task Sequence Media, the ability to run unattended is not enabled. So you power up your device to see the 'Welcome to the Task Sequence Wizard' screen.
Normally with boot media an engineer would be on site using the ISO but there are occasions where the boot media could be used and engineer involvement is not required. So you need the ability to suppress the wizard.
To do this check the 'Allow unattended operating system deployment' check box on the first screen of the Create Task Sequence Media Wizard.
If you don't want to recreate the ISO you can extract the SMS\data\TsmBootstrap.ini file and edit it.
Change the line Unattended=false
to read Unattended=true and save the file back to the ISO.