Reference for Yevamot 217:4
אמר אביי היינו טעמא דרבי אלעזר מספקא ליה אי מיתה מפלת אי נשואין הראשונים מפילים אי מיתה מפלת הא רמיא קמיה לייבום אי נשואין הראשונים מפילים הא עמדה עליו שעה אחת באיסור
Because there was a period when she was forbidden to him.<span class="x" onmousemove="('comment',' Lit., 'she stood for him one hour in prohibition'; i.e., at the time she was divorced she was forbidden to him under the penalty of kareth as his 'brother's divorcee'. Her subsequent remarriage does not alter her status. ');"><sup>12</sup></span> Said the Rabbis to 'Efa: If so, <i>halizah</i> also should not be required!<span class="x" onmousemove="('comment',' As any other 'brother's divorcee'. ');"><sup>13</sup></span> And should you reply that the law is so indeed; surely [it may be pointed out] it was taught: In the name of R. Eleazar it was stated that she does perform <i>halizah</i>! — In truth, said 'Efa, the reason of R. Eleazar is unknown to me. Abaye said, This is the reason of R. Eleazar:<span class="x" onmousemove="('comment',' For forbidding to the levir his brother's divorced wife despite the fact that at the time of his brother's death she was married to him again. ');"><sup>11</sup></span>