Why did factory workers dread unemployment?

It reduced the power of the unions to bargain for the workers.

Unemployment insurance was not available.

The unions would not support the workers.

Factory workers dread unemployment because it reduced the power of the unions to bargain for the workers. When there's a huge glut of excess labor union power dies because the unions can capture no more jobs than what are available. This leads to feather bed some more but the number of those is limited to less that what breaks the organization when it is not competitive anymore.