The short version is that Lathander decided to gain control of the entire Faerunian pantheon and failed, possibly as an attempt to eradicate evil from Toril. It took place around 800 DR.
A side-effect of Lathander's failure was that Moander infected Tyche, the neutrally-aligned "lady luck", with his corruption and the only way to "save" Tyche was to split her in two, thus producing Beshaba, bad luck, and Tymora, good luck. I think that Selune was responsible for this splitting.
Cheers
NPP