I wonder why the background was a blur in this case.
Having a larger focal length (and thus narrower FOV) will also correspond to a shallower depth of field (hence, background blur).
One thing to add to @saltyminty is it is not only the background blur, it is everything not in the depth of field will get blurred. (i.e., if there is something in front of the depth of field, it would also get blurred out)
There are three factors that affect the range of depth of field: aperture size, focus distance, and lens focal length.
Aperture: Generally speaking, the smaller the aperture, the greater the depth of field; the larger the aperture, the smaller the depth of field, given the same focal length and shooting distance. Usually we will use a small aperture to obtain a large depth of field to show the depth of the scene. When taking landscape photos, a small aperture is often used to highlight the subject and also to show a clearer and more expansive sense of space.
Focus distance: The last factor affecting the depth of field is the distance from the subject. The focal distance, also known as the object distance, it refers to the distance between the focal point and the lens, not the distance from any object in front of the lens to the lens, because only before and after the focal point can form a clear area. The impact of the object distance on the depth of field is expressed as follows: in the case of constant focal length and aperture, the depth of field is smaller when the scene is closer to the lens; the depth of field is larger when the scene is farther away from the lens.
Lens focal length: In the case of the same shooting distance and aperture size, a relatively short focal length can create a greater depth of field, emphasizing the characteristics of the surrounding environment, and intake of more environmental elements. Longer focal lengths create a smaller depth of field than shorter focal lengths, thereby capturing sharper close-ups.