Files in the deployment folder are locked when the app is running. Locked files can't be overwritten during deployment. To release locked files in a deployment, stop the IIS App Pool you can use PowerShell to drop "app_offline.htm".
$pathToApp = 'PATH_TO_APP'
# Stop the AppPool
New-Item -Path $pathToApp app_offline.htm
# Provide script commands here to deploy the app
# Restart the AppPool
Remove-Item -Path $pathToApp app_offline.htm