Sources close to Luann tell TMZ ... Tom D’Agostino didn't get along with Luann's family and it seemed his inappropriate behavior in the past was too much for the family to get over once married.

She made the shocking announcement Thursday on Twitter, saying, "It's with great sadness that Tom & I agreed to divorce. We care for each other very much, hope you respect our privacy during this sad time!"

Luann -- who got married just 8 months ago -- had a tumultuous relationship with 2 of her 'Housewives' co-stars, Ramona Singer and Sonja Morgan. Luann constantly bickered with them because Ramona and Sonja would bring up the fact they used to date Tom. Awkward.

Luann and Tom started dating in late 2015 and have no children.