That could have been automatically triggered by some kind of dead man's switch.
> This page exists only to help migrate existing data encrypted by TrueCrypt.
>
> The development of TrueCrypt was ended in 5/2014 after Microsoft terminated support of Windows XP. Windows 8/7/Vista and later offer integrated support for encrypted disks and virtual disk images. Such integrated support is also available on other platforms (click here for more information). You should migrate any data encrypted by TrueCrypt to encrypted disks or virtual disk images supported on your platform.
This could be because Windows 8.1 was only considered a relatively minor upgrade to Windows 8 by the author, or because it's existence was unknown at the time when the text was written.
This remains open to speculation, but it does not refute the possibility of a dead man's switch.