My interpretation was that the authors received a National Security Letter and chose to shut down development rather than let their software get backdoored. IIRC the shutdown announcement cited the discontinuation of Windows XP as why the software got discontinued (when it was cross platform and supported newer versions of Windows) and included a step-by-step guide for how to migrate to Bitlocker (a red flag for anyone remotely cynical).

An independent audit of the last version of TrueCrypt was published about a year after the discontinuation. It did not find any significant security issues or backdoors.