The biggest? So many reasons to choose from:

When the phone rings and I see the call is from the chief pilot, my first thought is, what on earth could I have done wrong that would make them call to fire me?

When I hear that a FA has cancer and will be starting Chemo, I know that she is about to get a checkride, that she will not pass, and I will never see her or him again.

When the company wants a broken plane moved and the pilot does not think it is safe under the conditions, I know he will either take it or get fired.

If the owner has a chance to make money moving all the planes from west coast to east coast, or by giving all the planes to someone else, or whatever, how the decision will affect pilots is not a consideration.

The airline is mostly unorganized and untrained. Decisions are made at higher levels but when things turn bad the blame is pushed to a much lower level than where it was made. Not to say that everyone is not in jeopardy, they go through VPs of Ops faster than pilots.

When a pilot gives 2 weeks notice his CASS is turned off before he finishes his trip and I know they have also turned off medical coverage before the end of a person's employment.

The union is worthless, there is no protection, the company does what it wants and the pilots take it. Good guys but no experience and no power.

I guess the last would be the biggest.