You need to read up on the legislation regarding your activity and location. I'm unaware of revoked undergraduate degrees affecting graduate degrees on a chain reaction. While your BA might have been a requirement at the time in order to receive your MA, it's been already fulfilled and it would require a separate procedure to revoke it, too.
Most jurisdictions would protect your job based on either the acquired rights principle (such changes cannot have retroactive effects), the reasonability principle (disproportional punishment to the misdeed), or the notorious knowledge principle (your aptitude havehas been publicly recognized). In my jurisdiction, even undergrad students who committed fraud in the acceptance process would not have their studies fully revoked, although the institution was allowed to expel them.
Therefore, I would be highly skeptical about having your BA revoked: it would have little effect (alone) on your career, it's likely to be overturned in most courts, and taking such extreme measures without strong evidence of gross misconduct would reflect quite poorly on the institution. Honestly, I cannot see why they would jeopardize their reputation in such manner, instead of just taking you to court, where they could build a case and cause more damage to your career.