Just when I thought there was peace and we'd resolved one issue, this happens.
The dialogue is subtitled at the video at 03:33. In the above sentence, I think this happened should have been there in place of this happens. Can you explain why this happens used there?
In narrative, especially spoken narrative, the present tense is often used to make the events more real and immediate, even when they refer to past time.
This is called the Historic present (or historical present).
"Just when ..., this happens!" is quite normal in such contexts, and does not refer to present time. "This happened!" would be equally possible, but would not serve the rhetorical function of bringing the event close to the hearer.
Edit: Speakers will happily switch between past and present in the same sentence. Here, "Just when I thought ... " isn't part of the narrative, but just setting the scene, and then the speaker shifts into the present for the incident that happened.
The speaker could have made the thinking more immediate and put it in the present: "So I think, There's peace now, and then this happens!" But in a clause after "when", the present is unlikely.
First of all, the subtitle should read "Just when I thought there was peace..."
Both of those sentences would be grammatically correct, but they do not mean the same thing, exactly.
Just when I thought there was peace and we'd resolved one issue, this happens.
This usage is in present tense, so the sentence refers to something going on right now. Better example, with context.
Context: It just started raining
Just when I get my tools out, this happens!
It could also be used in prose.
When you pull this lever, this happens.
Using the past tense makes this a story, so to speak. It happened in the past. It has already happened...
Just when I got my tools out, this happened [it rained]
When I pulled the lever, this happened.
After the dog bit my toes off, this happened.
Backing up to the "this happens" explanation, I realized that I used the present tense and probably caused some confusion.
Just when I thought
This, alone, is a statement of time for something to happen, in present or past. It can be rearranged to look like this
This happens just when I thought [...]
You need timing to separate the past from present.
