Wanted to get on my laptop but my phone will do
So Sascha has been Serena's hitting partner for 8 years now. Serena considers him like family she said (in her documentary with Venus) and they love each other platonically like a lot. She always confides in Sascha and tells him everything about herself if she's down, if she needs motivation before a match, etc (I wish I was on my laptop I j could've linked videos). They are nearly inseparable. However, starting in 2014 there have been talks in the Serena camp about possibly letting Sascha go. Most people think is Morataglou's fault because he's "brainwashing" Serena with his dick

. He also apparently cheated on her but she's so love struck she's overlooking that or whatever. I'd provide sources but again I'm on mobile
So moving on, yeah when Serena started working with Pat he was only a consultant at first, until they started getting cozy with each other. Now he became her full time coach and they are winning slams left and right. One of the biggest keys to winning past 30 is her ability to stay focused and The less peoe she has in her team, the easier it is for her to focus apparently. This year at the AO was the first time EVER she's won a slam without having her mom, dad, sister V, Sascha, or Esther in the stands. Literally only Patrick. Many people think he advised her to get rid of them all. Sascha said the reason he wasn't in Australia was because he was "sick" but no one believes that. He was sick before in slams but still was by Serena's side. Then he was replaced by a new hitting partner. Serena commented and said she loved him so much and that it'll be hard to figure out what to do with Sascha. Well he was kicked (and picked up by Vika). So we know that story
IW was a place she'd vow never to go back to because of the racist taunts she has received in 2001. However ever since she brought PM to the scene she's been thinking about returning, almost did last year, and finally did this year. Ever since he came she's been very official like in conferences, news reports (like the report she was coming to IW 2015) etc and she doesn't seem organic. Kinda like Sharapova.
She got rid of a family member with Sascha and now Victoria snatched him. There was a joke in the Serena documentary that Sascha joked with Ivanović about leaving Serena to work for her and Serena was SOOOO pissed he joked about that she made him pick up balls over and over again (she had them in her racquet and flung them wherever she wanted). So the fact he was left go so easily means she wasn't herself in the decision. Now that Vika has him, she'll know all of Serena's secrets
http://www.dailymotion.com/video/x2io121
That video is a summary of many Serena fans feelings in Twitter lol
So when Vika starts regularly dragging Serena since she will know all of her secrets that Sascha gives it'll be awkward for all parties involved.
Serena is so adamant on creating history she's leaving behind the people who helped her get there

. But she's a grown woman, she knows what she is doing