The group is experts in phishing attack -targeting employees of the organization with a fake emails. We believe hackers used the same method for compromising Forbes' employees also.
It appears they have gained admin access to the wordpress panel that allowed them to post stories.
The group appears to have compromised one twitter account of forbes (@forbestech) and two twitter accounts(@thealexknapp, @samsharf) belong to their employees. At the time of writing, Samantha sharf account still shows the hackers tweet.
The hackers said the reason for hacking forbes is because the publication posted many articles against syrian electronic army, with muchnhate for syria.