In my experience it has several meanings.
One is when you are burning your plant. ie temps towards the top of the plant are high (40c+). If I turn on my 400w hps in my small cupboard and not turn on ventilation , within minutes top leaves point up. Theu may point down soonafter , burn and eventually die but for a liitle while they point up.
Another reason is Mg deficiency. This is well documented on this site.
Another possible reason is Nute Burn. (not sure on this one)
Another reason is aggressive trainning. In cases where I twisted and pulled down major branches over and over to minimise height this happened too.
Also , in cases when your plant is a devout Christian or jew or Moslem, leaves pray on Sunday, Saturday or Friday respectively ...
)))))