Morning drop was obviously short-sellers' raid. The same drop was today on NTI and yesterday on CVRR. They are testing waters because the whole market is slowing down and refiners are going through consolidation after big gains.
They know, that there are people who have very big gains and may want to secure them by selling their holdings and at the same time there are not many new buyers - most people now worry that general market is too high and may go through correction = ripe situation for short sellers.
This type of raids is done by professional traders. They gang on selected stocks and drive them down. It's sickening experience, but what can we do.
I am sure article Time to Lighten Up On The Refiners - listed on CLMT, TSO, VLO page Mar.18 - also added to shake up refiners.