I am afraid there are no reliable theories supported by notable scholars. In fact, many authors - such as A. Scherbakov in his "Russian Political Emigration" point out their confusion considering the location of the murder, Mexican city of Coyoacan, wasn't exactly known as a hub of alpinism.

There's a lot of speculation out there, but most of it seems to be really reaching

Edit: as to why not just shoot him - Mercader intended murder to be quiet, which failed spectacularly, not to mention, he actually was in Trotsky's residence which was guarded at the time, thus unlikely to be let in with a gun.