Peeing on pad vs. marking in the house

We adopted a now 6-month old Pomeranian. We live in an apartment, so we've been training him to use a potty pad, and he's done very well with it. He also gets a daily walk where he will occasionally pee.

However, he recently started to mark pee around the apartment. It's clearly a different behavior because he will mark pee and then minutes later walk to the pad and squat and pee. It's in different spots throughout the apartment.

Why is this? And is there a way to train this behavior to stop?