Like a stable time loop, but when each version of the time traveller goes back in time, they create a history that differs from the previous alternate history? Is there a general term for such an occurrence in science fiction?
This is using the Many-worlds interpretation as a solution to temporal paradoxes. From the wiki:
The many-worlds interpretation could be one possible way to resolve the paradoxes that one would expect to arise if time travel turns out to be permitted by physics (permitting closed timelike curves and thus violating causality). Entering the past would itself be a quantum event causing branching, and therefore the timeline accessed by the time traveller simply would be another timeline of many. In that sense, it would make the Novikov self-consistency principle unnecessary.
Also mentioned more in depth in the main article about Time Travel:
A variation of Everett's many-worlds interpretation (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel.
The situation described in the question is most closely approximated by the Star Trek Voyager episode The Year of Hell S04E08.
Obsessed with restoring his species to greatness no matter the cost, a military temporal scientist creates changes in history that all but destroy Voyager.
The situation requires both the ability to time travel and for the traveler to be obsessive enough to continue to change history over and over.
TVTropes Time travel page gives a good categorisation list for various types of time travel. Given each time travel event leads to a new history and these histories are each unique and non-repeating, then as Don has already said you're looking at Alternative Timelines as a high level description. But you then need character tropes, rather than time travel ones, to explain why the timeline is continually changed.
Why does the traveller need to keep going back in time? Are they trying to arrange for perfection? To get back to their starting point? Make one important change that is so far elusive? Is there any potential for breaking the cycle? None of these things change what the "time loop" is called. But they significantly change the story.
It combines two concepts: time loop and plastic time. Plastic Time with high resistance avoids creating Alternative Timelines all over the place. The basic concept is history is alterable...up to a point. Past that point an alternate timeline is created "breaking" the time loop.