Dubbed Who Deleted Me on Facebook, the app is available as a free download and it will tell you who deleted you from their friends list since you last signed in. The app will also tell you whether that person shut down the Facebook account, or whether he or she deliberately removed you from his or her friends list – that’s about as much as it can do to answer the “why?” question.
If you don’t want to install the app on your smartphone, you can always try the browser extension, which used to be a Facebook app.
Of note, that app has an obvious shortcoming that’s unavoidable: it can’t tell you people who have deleted you in the past. Instead, it’ll monitor your account and tell you when people delete you after you start using it.
To install Who Deleted Me on Facebook, follow the source link to get your desired version (iOS, Android or web).