Saeviomagy said:
2. Your DM is out to get you. I can't think of another reason (as a GM) that I would interrupt someone's stakeout, especially with something as non-plot related as "a bad guy who already beat us up and took our stuff".
As a DM, I tend to always have an "emergency combat encounter" waiting in the wings. Usually at some point early on in the campaign, I set some group/organization against the PCs, so that the PCs can expect repeated attempts by this group to kill them.
Then I stat up an appropriate group of NPCs to make for a good combat encounter, and leave it waiting in my notebook or on my laptop.
Pacing is a funny thing. A DM can try to guess how long the players will spend on any one aspect of a planned session. Good DMs will often guess correctly. But occasionally the players will fixate on something, and your pacing will be thrown askew. (I had one session where my players fixated on a large pair of double doors in a sunken complex. Eventually, after 15-20 minutes of them trying to figure out how to open them, I had to straight out tell them there was nothing on the other side of the doors but solid rock.)
Now say I've planned a particular session and, to my surprise, the front part of it takes far longer than I'd anticipated, leaving the party far away from my next planned combat encounter as the session nears its end. This is where my "emergency combat encounter" comes in.
In my mind, it's better for the game if I toss in an "unrelated" combat every now and again to keep things exciting, rather than letting the players spoil a session for themselves by fixating on something that soaks up all of the session time and leaves the players feeling like they've accomplished nothing at the end of the session.
YMMV, but I didn't think the goblin ambush was particularly out of place. I do feel for Dagger75, though. It's hard to feel like a hero when the goblins are always beating on you.