Dismissal while Rendering Resignation

I have submitted my resignation already effective in 30 days. After a week of submitting my resignation- by the way company has accepted it- I received a show cause memo and today I attended the ad-con or hearing. The violation happened on First week of May. My supervisor did not issue the show cause because I have verbal agreement with my Manager that I will never commit same mistake again and shall i commit same violation she would issue a show cause memo. I did what I promised to my Manager, I have never committed same violation again until now that i am just rendering my resignation. I was surprised that my immediate sup issued show cause. I feel like he just don't want to let go of me with clean record.
My question is , can they terminate me though 2 months have passed since I have committed the violation and with all that agreement with my OM?
And why did they issue it after i resigned- it is very malicious, can i file a complaint against our company once they terminate me?

They are within their rights.

Present your evidence that you had an agreement with your Manager.

And ask for proof of the second incident (if any).