The choice of the Palantir as an object to throw seems strange.
The Two Towers, in The Palantir chapter, has this passage (Theoden speaking):
'(...) oft evil will shall evil mar.'
'That many times is seen,' said Gandalf. 'But at this time we have been strangely fortunate'
And in the previous chapter, "The Voice of Saruman", we hear the consequences of Grima's strange choice:
I guess that, even if we had entered in, we could have found few treasures in Orthanc more precious than the thing which Wormtongue threw down at us.'
A shrill shriek, suddenly cut off, came from an open window high above.
'It seems that Saruman thinks so too,' said Gandalf. 'Let us leave them!'
So we have that Grima picked for an useless retaliation (throw a stone to Gandalf? Come on...) of all things the worst possible choice and this is only explained by saying in essence that evil doers will suffer from their same evil.
Is there some reference/background explanation to this apparently unwarranted act by Grima in some commentary by Tolkien? I think maybe in his letters, notes, or Christopher Tolkien's commentaries...