This sounds like what happened to me a while back. There's a socket that pops on from the bottom and the ball joint that it pops into is rubber. Seems like it dried up and just gets loose and pops out. Only way to fix it was to replace the entire mechanism. I did get away with just tying the thing together with some electrial wire, and it was still working fine when I got the replacement part in the mail. Toughest part is getting the wiper arms off.
Of course when mine failed, it was in a driving rainstorm at night and I was on the highway. Couldn't see after about 5 seconds, but was able to slow down enough and make it off an exit that I was near. Went slow and took the local roads home, the rain also slacked off after a while.