"but WHY do I have to start up again in safe mode ?"
OK, I'm guessing here, but I've seen it happen often enough:
The updates are installed
The machine restarts
The updates need to be finalised (sometimes Windows tells you this, and sometimes not - it just appears to have hung)
You think it's hung, so you either press the reset button or turn it off.
Turning it off while it's finalising the updates, screws up Windows.
When you try to start, it knows it's been screwed and offers to start in Safe Mode.