~snip~
As Csmiami observed, there is a lot of transactions in and out based on that ss you posted, but at this point it is impossible to say how without your permission that last transaction occurred. If your seed is in personal safe, and your phone is safe from malware, then some hacker is obviously found a way to use some kind of exploit in Android OS or in the model of phone you are using.
What you definitely did wrong was that you kept such a large amount in your phone wallet, and that you did not use a hardware wallet or cold storage.
When did you download your wallet?
3.3.8 is the latest version available for download by default since July 11 2019. If you downloaded your wallet after that date then most probably you got phished and downloaded a fake one.
OP is using mobile version of Electrum, and latest version for Android is 3.3.7.0 from July 3, 2019. According to everything he posted, we're pretty sure he didn't install a fake wallet - otherwise he would have been hacked long before April 3.