Why did my phone reboot after copying an so file to the system/lib folder?

I ran the following command and the phone automatically rebooted after copying. Why? Was it because the so file was in use?

$ adb shell 'mount -o rw,remount /system && cp /sdcard/libfoo.so /system/lib'

My phone is rooted and enters as root when issued the adb shell command. My Android version is 5.1.

I also tried the following commands and the phone still restarted.

$ adb remount

remount succeeded

$ adb shell 'cp /sdcard/libfoo.so /system/lib'