I've never heard of the introduction of a mortgage drastically dropping anyone's credit score. I guess it depends on how you define drastic. The largest drop one would see is if they already had an installment loan that's already almost all paid off (less than 8.9% utilization) and then a mortgage was added. They would have had a 30 point boost or so from the first loan, but the mortgage adding on would raise overall installment loan utilization to near maxed out (depends on the original balance of that first loan, though). A score in this situation could drop 20-25 points. Now, if a previous loan was absent from the credit report to begin with (or if it was at very high utilization), there would be no 30 point boost in place, so the scoring drop from the mortgage reporting would be insignificant.
OP, what took your score from the high 600's to the mid 600's? Just inquiries? Were the inquiries for new accounts? How many new accounts and how many inquiries per bureau in the last year? It's generally not recommended that anyone app for anything when a mortgage is approaching, so hopefully you didn't get too crazy. Definitely don't app for anything any time soon.
Also to answer the question regarding potential AA that the OP posed, it's quite unlikely that he'd experience the closing of any cards due to a score drop based on his profile. 9 out of 10 times that a creditor takes AA against someone it's because of the introduction of a negative piece of information to a credit profile. This is almost always a first late payment and it's a huge sign of risk. You're showing the lender that you have trouble meeting your financial obligations and other lenders can see that reported late payment and take action. Score drops outside of the payment history sector of the FICO pie are far less meaningful. Even someone going from great utilization to maxed out utilization that may see a 100 point score drop is unlikely to see AA so long as they are still making on-time payments. I know someone that's had 8 out of 9 of their CCs maxed out for almost a year now. Their aggregate utilization sits at 93%-95% depending on the month. Not one of the creditors has taken AA against them. As long as you're making minimum payments and they're getting their money as agreed, usually things are fine.
