What speakers were they and what do you mean by "upside down"? Were they single monitors or used in pairs? Floor monitors or flown, such as for side fill?
There are sometimes reasons to hang speakers upside down in sound systems, although there is apparently a lot of confusion as to why this is done. Some of the comments regarding mirror imaging and the relative location of the HF driver probably relate more to stereo near field monitoring applications in studios than they do to live performance systems. Unless the drivers are configured to have the same acoustic center (e.g. a coax design with aligned drivers) then a speaker or array can only be "time aligned" at one point in space and relative timing between drivers will differ if you are not directly on whatever the manufacturer considers to be both the vertical and horizontal axis of the speaker. This is relevant to most studio environments with a defined listener position and nearfield monitors, but I doubt that the slight differences in the 'sweet spot' or imaging that would result from flipping the speakers are really that much of an issue in many stage monitor applications, especially if mono monitors.
On the simplest level, some floor monitors have different slopes on each side of the box to allow for it to provide different angles, so it may simply have been done to get the desired up angle. There are also speakers with asymmetrical patterns where some specific applications work better with the speakers flipped.
For theater and stage systems a speaker may be mounted upside down in order to get the LF driver closer to the ceiling to prevent combfiltering from undesired reflections off the ceiling. In other situations it can be just getting the HF coverage to clear a physical obstacle (light, duct, sprinkler head,
etc.) that would interfere if the speaker were hung in the 'standard' orientation.
Some manufacturers also use inverted speakers to address arrays. I know of at least one manufacturer that recommends turning the center box upside down in a three wide array of some of their speakers in order to obtain the smoothest response as an array.