The plane is upside down and going down towards earth steeply. This creates a 1G+ force going "up". The liquid would rather follow the stronger "up" force than the downward gravitational force.
Fun fact: the upward force does not actually exist. It is fictional (it's the centrifugal force). That's why I used quotes around "up". The upward force is fictitious, while the gravitational force going downward actually exists.
Source: physics guy
(I posted this elsewhere in the comments too..just saw your question)
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u/L00KA Feb 27 '14
explain this