Because that is the 0 date for UNIX time which is the standard Windows adopted also. Dates are kept in seconds from Midnight, January, 1, 1970. If the date ends up not getting set it defaults to 0 which gets translated to December 31, 1969.
The reason for the dates like that is when AC and dchan changed the software (remember the old look?) for the forum, the new system reset all of the dates. Those members have not visited EpicSki since the software change, thus have not reset their visit date. It is kind of the same reason taht everything posted on the "old" site shows all members as "guests" and not their compete information with links and such. I liked the results of the change because AC updated and combined two accounts that I had created for posting here.
Better than that, Skidbump, PSIA introduces the requirement for maintenance re-certifications every 5 years, and that has resulted in huge satisfaction levels amoung guests with american instruction! That happened in 2006 (Rocky Mountain Division introduced it, from memory), and instructors were able to use this as leverage to improve pay and conditions in following years.