I had the same on my Metallica with the gravemarkers droptargets.
There should be a spring which adjusts, how tight the droptarget sits on the playfield. If it isn't tight enough the target will accidentally drop (just like in your case) and if it is too tight it will just go down hardly.
By making the spring a bit tighter you should no longer have that problem.
Don't forget turning the game off before servicing!