If I remember correctly, the 4.4.4 update was also supposed to include just security fixes that would fix the problems with the HTC M8 Harmon/Kardon editions on Sprints network. (Correct me if I'm wrong though)
Either way, it's probably just to fix something that wasn't done right in the 4.4.3 update; maybe a new security hole came up and they had to quickly close it. That would also explain the quick release of 4.4.4, what, not even a month after releasing 4.4.3?
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Update: the 4.4.4 updates seems to fix a bug related to OpenSSL and the holes that that has had for the past month.
4.4.4 June 19, 2014
CVE-2014-0224 fixed; bug relating to OpenSSL