Altimeter failure. The instrument indicates a much lower altitude than
the real one. So, thinking they are much lower, software ejects the
parachutes, then when the rockets are turned on, the altimeter tells
that they have landed and software shuts down the rockets.

Then it goes from there till the crash at 300Km/h with no parachute and
no rockets...

Single point failure. All mission relies on the altimeter.