It could be a "you infringe on as many of our patents as we do infringe on yours" thing too.
I expect a counter-suit.
Regardless of how patent-heavy mobile devices may be, this represents a major change in Apple's policy.
In some discussion thread in the distant past, someone pointed out that Apple has never used its patents offensively. And a quick check through Wikipedia seems to confirm it: The only mention of patents was in cases where Apple had first been sued for violating a competitor's patents.
http://en.wikipedia.org/wiki/Apple_Inc._litigation
Up until now -- well into their reign in the mobile device sector -- Apple could claim that they were only amassing patents as a means of defending themselves from other companies with patents. Hence, this preemptive strike is unusual.
Obviously Apple and HTC have been in discussions (lawsuits are incredibly expensive and always a last resort) and the fact that Apple went ahead and filed indicates that these discussions failed. This means either HTC was offering less than 'market' in licensing fees, or Apple was asking for more. Which do you think is more likely?