Regarding applying to another university for PhD during the first year PhD program at a top research US university I've received my Masters from one of the best universities (may be the best actually) for my area. Moved to another top university as my professor moved. 

However, during this process I realized that my advisor can do a better job at handling the lab and careers of the graduate students. The current graduate student is in depression because of the work pressure and never spoke about it until recently. PI had instances of graduate students leaving the group because of the same issue. 

The project I'm working is just fine and doesn't seem to be worth all the mental turmoil. Feeling a bit uncertain about the future of PhD with the PI. 

Thought of transferring to another lab, but certain collaborations of the PI with other profs wouldn't facilitate that. I'm left with options of a change in the area of interest or move to another major or university. 

So, for me to apply to another university or another professor, the major concern is the reason behind the move. I'm not quite liking the toxic environment in the lab and the way the PI advises and want to move out of it. However, the concern is that I do not want to mention about this directly in my SOP and in my meetings with other professors. As this might reflect badly on me in return. So I would like to know what is the best way to deal with this situation. Thanks.
Although more experienced heads may give alternative advice, my recommendation is to focus on what you want to research rather than the personal relationship reasons you have. 

Remember what the SOP stands for: Statement of Purpose, why are you pursuing a PhD?

Professionalism aside, ad hominem attacks are not the subject of interest when you meet with potential advisers: you are the subject, what you are interested in, where you’d like to go, how you would approach a problem or issue.