A few minutes ago, the system switched to SlowKeys while I didn't even touch the Shift key!

Note: It sometimes happens that I unintentionally hold the Shift key for 8+ seconds (e.g. because I intend to type PageUp or PageDown, but don't do it immediately). In such a case, a message is written to the /var/log/Xorg.0.log file. But here, nothing was written!

Here are the last messages in the /var/log/Xorg.0.log file:

[778009.172] (II) XKB SlowKeys are now enabled. Hold shift to disable.
[870684.606] (II) XKB SlowKeys are disabled.
[870684.620] (II) XKB SlowKeys are now enabled. Hold shift to disable.
[870793.988] (II) XKB SlowKeys are now enabled. Hold shift to disable.
[870793.988] (II) XKB SlowKeys are disabled.
[970681.038] (II) XKB SlowKeys are disabled.
[970681.101] (II) XKB SlowKeys are now enabled. Hold shift to disable.
[970710.197] (II) XKB SlowKeys are now enabled. Hold shift to disable.
[970710.198] (II) XKB SlowKeys are disabled.
[970782.228] (II) config/udev: removing device  USB OPTICAL MOUSE
[970782.519] (II) evdev:  USB OPTICAL MOUSE: Close
[970782.641] (II) UnloadModule: "evdev"
[970783.017] (II) config/udev: Adding input device  USB OPTICAL MOUSE (/dev/input/mouse0)
[970783.056] (II) No input driver specified, ignoring this device.
[970783.056] (II) This device may have been added with another device file.
[970783.056] (II) config/udev: Adding input device  USB OPTICAL MOUSE (/dev/input/event3)
[970783.056] (**)  USB OPTICAL MOUSE: Applying InputClass "evdev pointer catchall"
[970783.056] (II) Using input driver 'evdev' for ' USB OPTICAL MOUSE'
[970783.056]    Option "XkbRules" "evdev"
[970783.056]    Option "XkbModel" "evdev"
[970783.056]    Option "XkbLayout" "us"
[970783.056]    Option "_source" "server/udev"
[970783.056]    Option "name" " USB OPTICAL MOUSE"
[970783.056]    Option "path" "/dev/input/event3"
[970783.056]    Option "device" "/dev/input/event3"
[970783.056]    Option "config_info" "udev:/sys/devices/pci0000:00/0000:00:1d.7/usb8/8-3/8-3.3/8-3.3:1.0/input/input19/event3"
[970783.056]    Option "driver" "evdev"
[970783.056] (**)  USB OPTICAL MOUSE: always reports core events
[970783.056] (**) evdev:  USB OPTICAL MOUSE: Device: "/dev/input/event3"
[970783.057] (--) evdev:  USB OPTICAL MOUSE: Vendor 0x15d9 Product 0xa4c
[970783.057] (--) evdev:  USB OPTICAL MOUSE: Found 3 mouse buttons
[970783.057] (--) evdev:  USB OPTICAL MOUSE: Found scroll wheel(s)
[970783.057] (--) evdev:  USB OPTICAL MOUSE: Found relative axes
[970783.057] (--) evdev:  USB OPTICAL MOUSE: Found x and y relative axes
[970783.057] (II) evdev:  USB OPTICAL MOUSE: Configuring as mouse
[970783.057] (II) evdev:  USB OPTICAL MOUSE: Adding scrollwheel support
[970783.057] (**) evdev:  USB OPTICAL MOUSE: YAxisMapping: buttons 4 and 5
[970783.057] (**) evdev:  USB OPTICAL MOUSE: EmulateWheelButton: 4, EmulateWheelInertia: 10, EmulateWheelTimeout: 200
[970783.057] (**) Option "config_info" "udev:/sys/devices/pci0000:00/0000:00:1d.7/usb8/8-3/8-3.3/8-3.3:1.0/input/input19/event3"
[970783.057] (II) XINPUT: Adding extended input device " USB OPTICAL MOUSE" (type: MOUSE, id 12)
[970783.057] (II) evdev:  USB OPTICAL MOUSE: initialized for relative axes.
[970783.057] (**)  USB OPTICAL MOUSE: (accel) keeping acceleration scheme 1
[970783.057] (**)  USB OPTICAL MOUSE: (accel) acceleration profile 0
[970783.057] (**)  USB OPTICAL MOUSE: (accel) acceleration factor: 2.000
[970783.057] (**)  USB OPTICAL MOUSE: (accel) acceleration threshold: 4
[992084.496] (II) XKB SlowKeys are disabled.
[992084.515] (II) XKB SlowKeys are disabled.

SlowKeys got enabled for no reasons around 992000. I could disable it by holding Shift for 8+ seconds.