Mac mini wakes 2 seconds after going to sleep
Its possible that a USB device (keyboard, mouse, external USB hard drive, etc) is not behaving. 

If you have other USB-connected devices, does this still occur when they are disconnected? You don't mention any additional USB devices being connected, so I'm assuming only a keyboard/mouse combo.

Disconnect all USB devices but the keyboard/mouse then type the following command in terminal. Once typed in, disconnect the keyboard and mouse:

sudo shutdown -s +2

Enter your password, (because sudo needs it,) and the Mac will sleep in 2 minutes. If it stays asleep, then the keyboard and/or mouse are not at fault. Try again with just one of the two devices attached to see which one is the culprit.

If the keyboard/mouse don't seem to be at fault, then test again with other devices connected &amp; narrow it down.

Secondly, Bluetooth keyboards and mice can also wake the Mac from sleep:

In System preferences > Bluetooth Advanced... section: