Run PowerShell script with different credentials

I was trying to run a PowerShell script with different credentials and for some reason it wasn't quite working.
This is what I was trying to do:

$secpass = ConvertTo-secureString "password" -asPlainText -Force

$cred = New-Object System.Management.Automation.PSCredential ("username",$secpass)

start-process powershell.exe -Credential $cred -ArgumentList  ".\Install.ps1 -serverfile .\servers.txt " 

I would get this error:

Start-Process : This command cannot be executed due to the error: Logon failure: unknown user name or bad password.

The thing is that I had used the credentials to do an invoke-command and they worked fine.

invoke-command -Credential $cred -ComputerName $server -ScriptBlock { Invoke-Expression "C:\temp\Install.ps1 -serverfile c:\temp\servers.txt" } 

So I did a bit of investigating:

PS C:\temp> $cred.GetNetworkCredential()
UserName Password Domain
username password

Which lead me to two questions:

1. Where did I expect the domain to come from if I've not passed it in?
2. Why did it work before?

At any rate, the solution is fairly obvious:

$secpass = ConvertTo-secureString "password" -asPlainText -Force

$cred = New-Object System.Management.Automation.PSCredential ("domain\username",$secpass)

start-process powershell.exe -Credential $cred -ArgumentList  ".\Install.ps1 -serverfile .\servers.txt " 

So now we get:

PS C:\temp> $cred.GetNetworkCredential()
UserName Password Domain
username password domain

The script now runs fine