The Time Splicing of the Dark Portal was done by Garrosh Hellscream to bridge present-day Azeroth with the Draenor of 35 years prior.
Reasoning of the Time Splice[]
Garrosh connected the Dark Portal to an alternate parallel universe, bridging his own universe with that of a parallel universe whose orcs had not yet drunk Magtheridon's blood. Thus he did not alter any history of his universe.