Some more points...
I will relate the issue to older kites ....say up to 2006 - (I am trying to avoid saying 1 kite is better than any other)
These kites often moved back and forward when sitting at 12 - this movement was exaggerated on gusty days or when at the edge of the kite's wind range (top and more often at the bottom). The kite accelerates forward with lifting force, gets to a point where the wind is starting to act on the top surface which pushes the leading edge down - if this was too powerful for the other balancing forces (drag, and lift rotation), the kite tucked and fell out of the sky (they still do!!).
Design changes have mostly removed this instability (test comments like - handles gusts really well etc etc point to this). However if you notice your kite slacking the front lines a little and hunting back and forward near or at the top of the wind window - ...
1) Make sure U do not have too much depower on.
2) Move your bar to break the cycle of the kite.....
a) In lighter winds - Rapidly pump the kite by moving the bar down then up.
b) In stronger winds - rapidly pump the kite bar left / right/ left / right..(This ups the drag acting on the kite and will force the Leading Edge up - by dropping the kite back a little in the window... giving U the chance to..
3) Re-position the kite off to one side or the other..(as above 11 or 1 oclock).
When the kite is off to one side it can still luff but is more likely to fall off to that side - not tangle the lines - and a chance to recover your kite - easier with experience

Even when your kite has tucked - if you can try and yank one of the steering lines - this can help the kite recover to one side and not invert (=tangle the lines through themselves

)
BTW - cbulota's point on line lengths - is often the source of this type of problem with modern kites - where 1 kite is falling out the sky when others are not.
So there is the reason / theory - good luck with practice..
Cheers
AP..