https://discuss.getsol.us/d/10390-caja-file-manager/5
from this year, a budgie-caja specific thread ^^.
see @Harvey third one down:
"to my knowledge we never defaulted the budgie edition to use caja so I can only assume you installed it at some point."
@Sebastian furthers this with "When you click on Trash, caja opens because Solus doesn't yet ship a list of default applications for common MIME types. In this case, GNOME and Budgie will launch whichever comes first alphabetically (the name of desktop file).
caja-folder-handler.desktop comes before nemo.desktop and org.xfce.Thunar.desktop.
Shipping a list of default applications for common MIME types is on the milestone list for Solus 4.6."
Schrodinger's Caja? I have no Caja whatsoever on my install. maybe it crept in with an app.
Or stepped in before we got Engrampra?
Inquiring minds, etc.
Like @WetGeek said, use it if you like it. In settings you can always set the default file manager.