Management can fire you for anything. However, the union can in most cases get your job back. The following things they can get rid of you without progressive dicipline and the union will likely not get you your job back.
"In cases of:
1 Proven Dishonesty
2 drinking of alcoholic beverages while on duty
3 recklessness resulting in a serious accident while on duty
4 the carrying of unauthorized passengers
5 unprovoked assault of an employee or supervisory employee while on duty
6 selling, transporting or use if illegal narcotics while in the employment of the employer
7 willful, wanton or malicious damage to the employer's property
shall be dischargeable offenses without the necessity of a warning letter being in effect."