There are several reasons why a person would delete or "privatize" a tree.
For example, other than the reasons stated earlier, one may have recently discovered that significant portions of the tree are inaccurate and wish not to publish until they can sort it all out. Another reason may be that the tree to which you were linked was deleted and replaced with a newer, more accurate version. Lastly, that person, for whatever reason, decicded to delete their ACOM account altogether.
There are probably as many reasons as one can imagine. Just be imaginative and you may come up with a few of your own. Those would be just as valid as anyone else's.