VVK
If you allowed PM's it would save me cluttering up this site, as I and many others have attempted to answer this question many times over the years.
In a nutshell:
1. They were negligent.
2. They suffered from a engine runaway or control restriction, as described by Sqn Ldr Burke.
3. They were not where they thought they were. (Distraction, Visual Illusion, Navigation equipment mishandling or malfunction). Take your pick.
In future please read back or PM some of the regular contributors. They will point you to the right place. Save us going around in circles.