ScriptRunner times out after 30 minutes if the installation does not complete before then.
When it times out, it does not end the installation process causing MSIEXEC to become locked up. Because ScriptRunner has reported a failure back to the IME, it allows the IME to start its next app install, but because MSIEXEC is still in use from the previous installation, subsequent installations fail until the vendor's install eventually completes or times out.