It seems like Apple may be concerned that their initial complaint may not go in their favor and has determined that the patents in the new complaint are potentially stronger.
In April, the ITC staff expressed its opinion that HTC did not infringe any valid Apple patents within the scope of that investigation. The target date for a final initial determination by the Administrative Law Judge (ALJ) hearing the case is August 5, 201. The ALJ could still rule in Apple's favor: the staff recommendation is, after all, just an opinion. However, a negative ITC staff recommendation does not bode well to say the least. I'm sure Apple will fight hard to make the original complaint succeed but doesn't want to depend on the outcome of that case.
The title of the complaint is "Portable Electronic Devices and Related Software", which suggests that Apple may be targeting both HTC smartphones and tablets.