Why did UPS go on strike?

UPS went on strike due to issues concerning part-time vs. full-time jobs. 60% of the unionized jobs were part-time, part-timers make $11.00 per hour whereas full-time workers make $19.95 an hour. They also went on strike concerning the issue of control of pension and health funds. They wanted to reduce part-time jobs by combining them into full time positions.