Theory: Thrall was an Alliance plant intended to remove the Horde from the Eastern Kingdoms

Consider the following:
>Thrall is raised by humans for the first 15-18 years of his life. In that period, he only sees another orc once. Thrall himself admits that he was essentially raised as a human, with a human outlook on life
>Thrall miraculously "escapes" a heavily guarded fortress with the help of his owner's daughter - somehow Thrall, who would stand out for miles as the only orc in the area, is not seen by any of the large number of guards
>Thrall just happens to know where the Horde remnants are camped out and goes directly to their base with a Frostwolf clan banner that he just so happens to have kept on his person for his whole life, which wasn't confiscated or destroyed by his militantly anti-orc captors
>Thrall spends years living amongst the Frostwolves and worms his way through the Horde ranks, he tells Doomhammer exactly what he wants to hear and ends up succeeding him as Warchief
>Thrall and the orcs liberate all the internment camps with surprisingly little resistance from the Alliance, who would surely be able to quash such a rebellion considering they were at peace and surely energized from their victory against the Horde
>Once the internment camps are liberated, instead of declaring Total Human Death for the years-long subjugation of his people, Thrall declares his New Horde to be a Horde of peace and orders them to sail to the other side of the world

This is an absolute masterstroke of human scheming. The Alliance spent years trying to remove the orcs from its lands to no avail, only to realise that they could do it from within via a plant - Thrall. The Alliance spent nearly two decades educating Thrall exactly for this purpose, and he did not disappoint.

Were it not for Thrall, the Horde remnants would surely have united under the leadership of Doomhammer and Grom, liberated the internment camps and then launched a second siege of Lordaeron.