We have probationary periods where as an employer, you don't need a reason (although if it's based on anything other than employment issues, Hello Human Rights!). After the probationary period, you need to prove there's an issue and show steps to curtail it. We use a 3 strike rule at our company. Verbal warning, written warning, suspension/termination. Typically, these need to be completely different issues. Showing up late and getting a verbal warning doesn't mean you can move to written warning because someone didn't complete a job responsibility.
Of course, you can always fire someone for whatever you want, you just run the risk of them fighting you in court if you didn't follow the proper steps. Most employees actually don't know the full extent of their rights.
____________________________
Donate. One day it could be your family.
An invasion of armies can be resisted, but not an idea whose time has come. Victor Hugo