Start with a hypoxic pilot, that realizes that there is a problem, and in a semi-conscience state makes some close but no cigar decisions. He realizes he needs lower for oxygen, fixates on the number 10 thousand, and in his stupor does the math and comes up with 28 minus 10 equals 18 thousand, which he asked for, NOT the 10 thousand feet that he needed, with an immediate, emergency decent.
The aircraft was brand new. The ferry from France would indicate that the pressurization system worked.
A pinched seal would be insidious and could be undetected.
And if no one had checked if the crew O2 valve was turned on, well there you go.