External Transfers from savings accounts are counted as one of the 6 unsigned transactions allowed per the federal government's Reg D.
Reg D is a Federal regulation by the government that places a monthly limit on the number of transactions you may make from your savings or Money Market account without your physical presence at a branch or ATM.
Only savings accounts and Money Market accounts are regulated in this manner, and you can avoid meeting your limit of 6 by using your checking account as the withdrawal source for your External Transfer instead of your savings account.