All i can find on the Wiki is this
Kyubey tells Madoka to reveal her true powers in order to save Homura, but Sayaka tells Madoka not to listen to him. She stabs herself in order to summon Oktavia and an army of familiars from other witches.
Source: Sayaka Miki - Rebellion Story (5th Paragraph)
i can't find anything else on it so i will give my interpretation as to why Sayaka cuts out her own heart.
Leading to Sayaka's downfall in the final timeline pre-Madoka Kami1
- In episode 6 the Puella Magi learn that their bodies are in fact dead.
- In episode 7 Hitomi tells Sayaka that she has feelings for Kyosuke and out of respect for their own friendship and the closeness between Sayaka and Kyosuke she'll wait 1 day
- When Sayaka goes to hunt witches that night she confines in Madoka her deeper feelings about now regretting have saved Hitomi and that because of the state of her body she can't face Kyosuke or be loved by him
- The next day we see Sayaka watching Hitomi and Kyosuke. i suspect that this was when Hitomi confessed and despite what Sayaka said to Madoka she was holding out hope that if Kyosuke rejected Hitomi then it might be a sign that he was waiting on Sayaka thus he might accept her condition. however Sayaka knows that Kyosuke has accepted Hitomi's feelings and became her boyfriend
so right now Sayaka has had her heart broken and is giving into despair. some interpretations of this i have seen is having the heart ripped out of the body.
Come Rebellion Story i suspect that Sayaka needed something to trigger her Witch to come forth so she recreates the feeling of having her heart ripped out by stabbing herself.
I would like to say this would be the same for any Magical Girl with access to her Witch (recreating the feelings of despair they felt before transformation) however Nagisa is the only other Magical Girl who became a witch and went back to normal2 and she doesn't seem to show a despair trigger like Sayaka. But she may have already summoned her witch form when she took Charlotte's form and became Bebe
1: i ignore the other timelines as Sayaka didn't seem to become more powerful because of them unlike Madoka
2: Homura is something more so she doesn't count