Performance improvement plan

Hi, i am prasad. Working as a HR in retail company. Yesreday I got PIP letter from my senior. It was a shock to me. They have not discussed on what basis they are going to issue PIP. I am working here since last 1.5 year. Recently we had huge attrition.hence we are paying less salary we could not fill the gap. Same thing I had highlighted to management. In last 4 month there are 81 staffs left the organisation. I had recruited 64 employees in last 4 months. And I had got transfer to this location. There were 36 manpower gap was there. My senior also came to help me in terms of recruitment. Hence we are paying less salary she also could not recruit anybody. As well as I am alone handling 400,employees. As well as my colleague left recently I am performing her role also. Recently I have cleared interview in other company within two days I get offer also. But senior realised that I am about to leave the organisation. She suddenly issued PIP. It would effect me when my future company intiate back ground verification about me. Before issuing PIP they have not provided me any feedback and have not communicated that for what reason they are issuing PIP. Last one year my prformace was good . so they had given me increment of 15% and transferred to this location. As well in my recent appraisal also I got 2.96 out of 5 which is meet expectation. But they have issued PIP to hurt my career. Kindly let me know how to get legal remedy. Thanks Prasad G