The scoring campaigns might have triggers that do not account for the fact that the email bounced. Hard to say without seeing the scoring campaigns. I suggest you go into the Activity Log of a lead and see what happened - look at when the scoring fired, what is the scoring campaign and see if there is a fault in the scoring logic. Or you could set up a campaign to reset the score of a record if the email bounces.
There are two things going on here:
- Engagement Score - this is automated by Marketo based on an index of 0-100 for engagement of ALL emails from ALL clients. If your email bounces on a lead, then it still counts as "not engaged" and gets a score, prob below 50.
- Lead Score - unless your scoring system (as Alex noted), is triggering on Email Bounces IS ANY, I doubt your Lead Score is changing because of a bounce.
Marketo doesn't do much other than temp block the lead who bounces or possibly invalidates them. You should explore setting up list cleaning automation system.