Full facts
We don't have the full facts around this incident. It is also quite possible, as a result of German privacy regulations, that we will never get the full facts.
I can certainly conceive of scenarios where immediate firing would be appropriate: for example, if management had unimpeachable evidence of collusion between employees to subvert a safety process for personal gain. Note I am NOT saying this is what happened - just pointing out that there can be occasions where instant dismissal is the correct thing to do.
On the other hand, firing first before finding out the relevant facts is something to be discouraged. Sufficiently serious cases might warrant suspension pending further investigation, but summary dismissal should only be used where cases are legally watertight. Such occasions are quite rare.
I can well understand the all too common failing of forgetting to reassemble something. Someone I know had the interesting experience of having a wheel on their car changed by a garage and subsequently discovering on a motorway that the wheel nuts had only been made finger-tight, rather than fully torqued. This is why independent checks and test procedures are so important. If you are interrupted in the middle of doing something, it is easy to skip a step, especially if you are focussed on the goal, made worse if you are tired and/or under time pressure (which may be self-imposed). Plumbers have been known to forget to solder up joints that are subsequently overlaid by plasterboard and paint, and only when the system is first used is the mistake found. It happens. So there could well be many reasons why this failure occurred, so human factors experts do their best to design processes that make things easy for humans.
In short, my mind is open on this: there could be a good reason for summary dismissal, which we might never know: or it could be a visible failure in applying a just culture. We have no idea currently, and we may never know.