Interview for Secret

I submitted an SF-86 for a secret clearance during the first week of December. A week later, in mid-December, I received my interim clearance. During the first week of January, I was interviewed by an investigator for about an hour. Is this normal? I thought that interviews were generally only for TS clearances.

The clearance is for a position with a government, more specifically DoD (I believe), contractor.

All positions have risk levels associated with them Positions at moderate and high risk require interviews, so you could be in one of those positions that also requires a Secret clearance.