The sliding step is a PSM (parametric special module) function, so the disconnect of the Aux battery may be telling the PSM that something's hiccuped (which it may take as a shorted motor or damaged wire) ... so it may be engaging a safety fall-back of "don't try to move things until a known state returns" ... which the fuse-pull probably does.
added: reading the option book for the T57 electric step ... next time this happens, try opening and closing the sliding door *instead* of pulling the fuse ... if that "fixes" it, then it was the obstacle-detector going into "safe mode" that was the cause.
more added: according to that book, the only things MB hangs on the Aux battery are:
* Code E46, Power socket in cab
* Code EE3, 115 V Socket
* Code ES2, 12 V Plug socket in trunk / load compartment
* Code ES5, Charging package, dashboard
* Code V85, Smoker package
* Code E1I, Smartphone cradle
* Code E1U, USB-C Socket, 5 V
The ES5 and E1U do draw a little power, even when there's no load.
even more: if the isolation/cut-off relay were to fail in a "closed" (i.e. connected) position, then the Aux battery would be permanently attached to the starter battery, and reconnecting the Aux negative cable would have the two batteries instantly try to equalize their voltages. You'd get sparks similar to those when attaching a jumper cable (between two good batteries).
You can test for that last situation by using a voltmeter between the Aux battery's positive post and the Sprinter's metal frame (or any "starter-side" ground connection (usually brown wires)). With the Aux batt's negative cable disconnected, the Aux batt's positive post should not show a voltage (it should be "floating" (no loads at all) or downright zero). When the negative cable is reconnected, that voltage would hop back up to the battery's level.
If the isolation relay *had* failed in a "connected" state, then the initial voltage test would read the starter battery's voltage (via the failed relay).
--dick