The story as told by his teammate, Jorg Jaksche who was not far behind him when it happened and first on the scene after.

https://www.velonews.com/2019/07/tour-de-france/remembering-joseba-belokis-crash-on-a-hot-day-in-gap_497941

Seems their DS, Manolo Saiz, gave orders that were not to be disregarded that turned a calm moment of a stage that mattered little on a very hot day to a setup for what happened.  Jaksche told Beloki to go easy on the descent, that what could he gain?  10 seconds?  Saiz by contrast was apparently going nuts yelling "Venge, Venge!"  So Beloki went fast, faster that Armstrong, a very good descender was willing to go.  And we all know where that ultimately went.