Whenever you apply for new credit card accounts or other loans a hard inquiry gets put on your credit rating and drops the score by a little bit.
The score drops from the hard inquiry due to the consumer opening accounts and the reason it drops is if you open too many accounts at one time or apply for too much credit at one time then it's usually a sign that the person is not responsible with their credit and are likely to not pay back those loans or credit cards on time.
I don't think it should drop your credit score but it does and it's a way to prevent consumers from opening too many loan or credit card accounts at one time because the person will lower and hurt their credit score by opening up too many credit accounts when a hard inquiry is put on their credit rating.
Only open up credit card accounts or take out loans that are really needed and that you can reasonably pay back on time.