According to http://www.macrumors.com/2016/02/15/apple-to-fix-january-1-1970-date-bug-ios/:
Apple has not provided a reason for the bug, but YouTube video maker
and programmer Tom Scott speculates that setting the date close to
January 1, 1970, which is 00:00:00 in Unix time, may be resulting in
an integer underflow -- in this case, a date prior to January 1, 1970.
iOS then handles the underflow by returning the negative integrer to
the maximum value, which Scott says results in a date that is some 20
times longer than the universe is expected to last. Scott believes iOS
may have difficulties handling this large number, resulting in
affected devices crashing.