Here's why:
* "It" refers to an object that was previously mentioned, but it's unclear which object is being referred to. Is it the cap, the marker, or something else entirely?
To make the sentence clear, you could rewrite it as:
* "Remove the cap from the marker before you hand it to me." (This makes it clear that "it" refers to the marker).
* "Remove the cap from the marker before you hand the marker to me." (This explicitly names the object being handed).