I don't know about fraud alerts specifically, but you may be running your credit score into the ground (and you'll only realize it with a time lag). Applying for a number of cards at the same time isn't the issue by itself; applying then again just a month later is. The thing is, when you apply for a credit card, it puts an "inquiry" into your credit records. A number of inquiries over a short time makes for a red flag temporarily. Now, the inquiry doesn't show up immediately, which is why 6 cards over 2 days wasn't a problem at that time. But now it's a month later, and those 6 inquiries, 6 new cards, have all shown up, and so you trying to open lots of additional cards is throwing red flags all over the place. (And, in fact, they may not all have shown up yet; it may be even worse next month, as even more show up!)

These inquiriy "red flags" tend to drop off after a few months, so if you were opening another batch of cards but not until a few months from now it might not be such a problem. But the fact that you're doing it just a month or two after 6 were done at the same time, don't be surprised if even stranger and more unwelcome things start happening.

For example, some companies such as Chase are starting to refuse to award bonus points for customers who have a record of opening cards for just the bonus and nothing more. So just because they open a card for you is one thing, whether they'll give you the bonus is another. (And I would presume you have no need of so many cards if you're not getting the signup bonuses for all of them, right?)