"At Houston Galleria, they always call (as in they have for months) to do a pre-auth on my CC if I'm buying multiple GCs. Slight annoyance/slowdown but NBD. You can tell because they write the pre-auth number on the form you fill out."
That is called a Code 10 Call. They are suspecting you of fraud. They are guaranteed payment if the Code 10 Operator gives them authorization, though.
I think all retailers should practice the "Code 10 Call" on large gift card transactions. It may take longer but at least they are guaranteed payment.