It could just be changes with how Google measures stuff with Universal Analytics (I assume you weren't using it before?)... If the session times are shorter, but you end up with more sessions, it might just be a matter of changing the session timeout in your Google Analytics account. Either way, Universal Analytics measures things differently (with things like session unification), so it will never match exactly with the old non-Universal Analytics (again, they simply measure things differently).
Similar issue...you can see my Sessions screenshot from Google Analytics below (ih8mud.com, including forum.ih8mud.com) - the "jump" is while this plugin was installed. I've JUST reactivated and disabled the "attachment" option as noted earlier...curious to what results are in a few days.
Xenforo. Switched the XF plugin on again yesterday, and am monitoring whether it still causes the crazy session numbers.