"given that everyone was happy to keep using TrueCrypt up until 1 day ago"

The same was true for OpenSSL a few weeks ago.

One of the most plausible theories is that the TrueCrypt developers found a gaping security hole (ala OpenSSL) and realised that releasing a fix for it would reveal the bug and compromise every TrueCrypt partition in existence, so they chose to kill the project rather than risk the safety of all of that currently encrypted data.

If that scenario is true, there's a big rush to switch to something else before the bug is discovered by other researchers.