This happened twice to Walton; once during Friday's practice, and again in Saturday's warm-up prior to the race.  The problem could have been something as simple as a blown valve cover gasket.

This side view offers an opportunity to demonstrate the truth of some Scarab lore.  The legend:  Due to repeated front and rear impacts, the Scarabs became shorter and shorter as they progressed through the cycle of crash and repair.  Here's a side view of Harry Heuer's car at Riverside in 1961 --  note truncated nose & rear crash bar.