Routine checks could be triggered for a variety of reasons from what I've learned. If you have extensive travel history or have had jobs in countries that are deemed 'High Risk' by ASIO, a routine check could happen to your application. Or sometimes they just need to verify basic information such as work history. And people are saying that no one knows how long a routine check's going to take as IMMI outsources security checks to ASIO.
I called Adelaide GSM and the nice lady said she does not know how long it is gonna take but its quite normal for applicants to go through routine checks, and when the checks are cleared then the application can proceed.
It is annoying though not knowing how long it would take for ASIO to complete the checks....