BloodFeastMan
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.
You can change this behaviour beforehand, however. So, if you wanna change that caja opens when you click on trash, you can run in terminal xdg-mime default nemo.desktop inode/directory
if you want nemo to open and xdg-mime default org.xfce.Thunar.desktop inode/directory
if you want thunar to open.