Hey Wishy, I've got your answer!
Last year, the Disneyland monorail began making one-way trips back and forth between DTD and Tomorrowland only, and skipping the rest of its route. This is because they needed to use the monorail as transportation more than an attraction, to shuttle people in and out of the park when crowds got really huge. (Which, during the 50th, was pretty much all the time!
)
They also have been making one-way-only trips to avoid passing over the lagoon while it is being worked on. Making the one-way trips means that the monorail only has to pass BY the lagoon, and not actually over it, where guests could look in and see everything.
I don't know what to tell you about your past trips to DL, though, because it used to make the full circuit so there's a good chance you've ridden it over the DCA bridge!