You're making a lot of assumptions here and basing it on the limited knowledge of a specifies that has failed to make any significant strides into exploring anything other than its own solar system.
Sorry if I take "explosion makes rock go far" as being the more plausible explanation than "Aliens want to wait a million years for a brief glimpse at a distant solar system".
Of course, that is still less likely than some unusual rock doing the same thing.
Playing Devil's advocate, a probe that size would likely visit many star systems, the Solar being a random one in the bunch. The brief pass would be necessary to avoid the energy expenditure of braking and reaccelerating.