According to Wikipedia, the tree's unique shape reminded the Mormons of "a Biblical story in which Joshua reaches his hands up to the sky in prayer".
I am no biblical scholar, but I couldn't find this story.
The nearest I got to it was in Joshua 8:18, where God says to Joshua, "Stretch out the spear that is in thy hand against Ai." When Joshua complies, the Israelites are finally able to capture the Canaanite city of Ai.
According to biblemeanings.info, there are several references in the Bible to God stretching out his hands or arms, as a sign of omnipotence.
© Haydn Thompson 2017