The union and management were unable to agree on terms, and over 99% of the USWA membership voted to go on strike when the current contract expired.
[1] USX rejected the offer, as it had already begun lengthy and expensive plant shutdown procedures in anticipation of a strike; furthermore, the ability to stop work on two days' notice would have given the union the ability to cost USX even more money, by disrupting deliveries and requiring another expensive and unplanned plant-idling operation.
Management-level employees began loading trucks and trains with previously-produced steel kept in inventory themselves and shipping it to customers, which led to violent clashes with striking workers.
The union remained publicly neutral but internally came to believe that Icahn merely wanted to extract short-term cash from USX and was not really interested in the long-term health of the steel industry.
[1] By January 1987, USX was about to miss major orders for car manufacturers, and union members had reached the end of their unemployment benefits.