Question about dropping leaves

[butterfly_muse]

Morning, everyone!

I am curious as to what makes orchids drop leaves, in this case specifically phals. I've got a big NoID phal in my living room window and it's dropping its bottom leaf (which is it's now bottom leaf...it already dropped its very bottom one which was located on the other side of its crown). It started putting out a new leaf and then just kind of....stopped.

Next to it is a mini Phal. Timothy Christopher, who is also dropping its lowest leaf.

And then in my kitchen, a totally different part of the house, one of my mini NoID gift phals started putting out a new leaf, stopped growing it, and is instead dropping its lower leaf and growing new roots...

What are they doing and why? And is this something in my environment that I need to fix? Or is it because it was hot this week? My kitchen stays much warmer and more humid than does my living room where the other phals are, so I am confused.