"How this could happen" is he's an alcoholic, and this is what alcoholics do.
I'd strongly - strongly - suggest blocking his email address (your email provider can tell you how to do this). When he attempts to email you, it will bounce back to him and let him know it was not accepted.
Email harrassment is harrassment. If he doesn't stop and you can't make him, a restraining order may be necessary.
And if the credit card thing happens again, consider springing for an hour with an attorney to talk about your options and make sure your defenses against his actions are airtight. In fact, you may want to do that anyway. I know it reduced my blood pressure by at least 20 points to find out that I was in no way, shape, or form responsible for "his" debts in my state.
You may also find out from them how to protect yourself against him taking out a credit card in both of your names through one of those mail-in offers. My A sister did that and ruined her ex-husband's credit....so this may be a good topic as well.
I understand about being safe financially. This is how you can be assured that it's really your motivation - and the rest? This is his bed. You MUST let him lie in it or you can kiss your serene new life goodbye. You've worked too hard to get to this place to let it all go again.
Hugs to you, wantsout --