Jennifer Capriati's ex-boyfriend says he believes two big things went down in her life that caused her to accidentally OD on prescription drugs -- his return to porn and her depression over tennis.

Dale DaBone dated Jen from 2003-2009 -- if DaBone sounds like a porn star name ... that's because it is -- and DaBone has 443 films under DaBelt.

DaBone claims Jen got horribly depressed every time a grand slam event went down. Interestingly, the OD occurred Sunday, during the time Wimbledon was on. Jen would always complain she was forced out of tennis because of injuries and began taking prescription meds.

But there's something else that DaBone says sent Jen on a downward spiral -- his recent return to porn ... something Jen despised. DaBone signed on with Vivid Entertainment to star in "Batman XXX: A Porn Parody."

DaBone tells TMZ after Jen OD'd, he called her in the hospital Sunday night and she was "hysterical" over his decision to return to XXX films.