Saruman greatly underestimated Grima. I don't think that he knew how much Grima knew. For example, at the end of the Scouring of the Shire, Saruman mercilessly "teases" Grima about having killed, and perhaps even eaten, Lotho. Grima gets so angry that he kills Saruman. A very underestimated character indeed! I wouldn't put it past Grima at all to slink around Isengard and find out as much information about his master as he could, to store away in that slimy little mind of his. He must have found out about Saruman's "prized possession" and where it was kept, or the general area in which it was kept.
(I'm using the movie-Isengard for this next part, because the book doesn't describe the layout of Isengard much.) In the movie, a door connects Saruman's "study" to a balcony from where he addresses his Uruk-hai army. I'm assuming that will be the same balcony that Gandalf will break his staff from. In that case, Grima could have been waiting behind in the study, and as he became more and more incensed, could have seen the palantir, and lobbed it out of the balcony.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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