There is no medical evidence indicating that abortion causes breast cancer. There's one guy who teaches biology at a third rate business college, who claims there is, and a little chorus of politically motivated tagalongs who support his sorry excuse for research because it fits their political agenda. Real biologists and medical researchers, like the ones at Baylor University (not exactly a bastion of liberalism or pro-abortion activism) have debunked this nonsense thoroughly.
The level of "science" behind Brind's claims is several notches below the level of the laughable "science" behind Al Gore's wild global warming alarmism (note that Al and his supporters are also fond of citing lists of "scientists" who support their claim). But as long as it dovetails with anti-abortion ideology, conservatives who rightly guffaw at the global warming nonsense eagerly swallow the abortion-causes-breast cancer nonsense hook, line, and sinker. It's exactly the same mentality that motivates the anti-capitalism, anti-individual liberty crowd to tout the dire warnings of Gore and the sketchy scientists who provide him with his ranting points.
When a woman becomes pregnant, her body gears up to support the pregnancy. An abortion is an operation that suddenly shuts that down, causing physical and sometimes mental trauma. The effect is quite unlike a natural abortion. A therpeutic abortion can, therefore, not really be called therapy, and such interventions are compounded when the abortionists bitches the procedure. Does an abortion "cause" breast cancer? My guess is that only in the sense that a bullet wound might cause a stroke. Neither would is good for the victim. The main difference is that doctors usually don't shoot their patients.