Have had similar things happen in the USA. It's all due to "terrorist" precautions. Happened so many times over hear the Congress ended up passing a law to make it illegal for them to NOT provide water, air conditioning and restroom access. However they did not make it illegal to hold them on the plane, nor did they even put a time limit on it.
Supposedly, it has to do with, once they let you off the plane, they'd have to unload all the baggage, and then re-inspect all the passengers and baggage before reloading.
Don't flame the messenger, this is what has happened here in the USA.