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During Shuttle ignition sequence:

the engines always slightly gimbal closer together as they reach full combustion (I'm guessing, from the plume appearance). Why?

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Before the engines started but after the Auxiliary Power Units (which provide hydraulic power) were started, the engines were gimbaled to ensure that the thrust vector control system was working properly.

At T minus 4 minutes, the fuel system purge begins. It is followed at T minus 3 minutes 25 seconds by the beginning of the engine gimbal tests, during which each gimbal actuator is operated through a canned profile of extensions and retractions. If all actuators function satisfactorily, the engines are gimbaled to a predefined position at T minus 2 minutes 15 seconds. The engines remain in this position until engine ignition.

As explained in the 1982 Press Reference Manual (p. 132): "In the predefined start position, the engines are gimbaled in an outward direction (away from each other) so that the engine start transient will not cause the engine bells to contact one another during the start sequence"

Beginning at T minus 0, the SSME gimbal actuators, which were locked in their special preignition position, are first commanded to their null positions for solid rocket booster start and then are allowed to operate as needed for thrust vector control.

The first and last quotes are from the Shuttle Crew Operations Manual page 2.16-30 and 31.

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    $\begingroup$ You can see the startup transient about two seconds before the linked point in the video in the question. $\endgroup$
    – Mark
    Commented Mar 22, 2020 at 18:26

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