One reason for this could be lack of initializing AoE threat on targets. Marking kill order can possibly help to prevent this from happening, e.g.
/party Kill order is:
/party {skull}: highest kill priority
/party {cross}: second kill priority
/party {moon}: sheep target/lowest kill priority
Have a look at Elite’s Warrior Tanking Guide for some useful macros and other Classic Warrior tanking tips. You may find it useful.