Question: When I picked up my guitar today I noticed my 1st string was a bizarre 2 semitones flat. I would tune it but the instant I bend it, stretch it, or play for a minute the string goes back down to the exact same note and won't hold tuning any higher.
Granted the strings are 6+ weeks old and I play 6-7 days a week but this only occurs on one of my guitars. I'm using GHS 9's in drop C (easy on the fingers).

Why do the higher strings lose the ability to hold certain tunings? Is there a reason it always detunes to the same note? Is there a way to prevent this so I don't have to change my strings so suddenly?(I only want to change them when I record something high quality).

Answer: 
Old string do this sometimes. It may be the wrap around the tuning post is slipping, it may be the winding at the ball-end is unravelling, but after all those hours playing, and possibly(?) no cleaning after play means they're ready for changing. They are the sacrificial part of guitars, anyway. Answer to 2nd part - always clean and dry after playing, with clean dry hands, and storing guitar in a static temp. and humidity - away from interested tiny hands.
6 weeks isn't 'old' for a guitar string. Assuming that you've tried the obvious solution of a replacement string, perhaps you should be looking for a fault in the tuning mechanism.