I had this issue at one point because for some reason the game was trying to equip the weapon in my offhand slot, changed the sword/board macro to /equipslot 16 I believe for the mainhand and have no issues with weaponswap macros now.
Just made your macro in game, using equipslot16 for the mainhand, it works fine, startattack included and all.
#showtooltip Defensive stance
/startattack
/equipslot 16 Swinetusk Shank
/equip Aegis of the Scarlet Commander
/cast Defensive Stance