It could be due to a number of factors. Here are a few possibilities:
1) You have a lot of salt in your diet and your body is "purging" yourself of it, but this is not likely unless you really notice you are taking in a ton of salt
2) If you have Cystic Fibrosis, your sweat will be 2-3 times saltier than normal
3) Some people are naturally just "salty sweaters". This is actually a current issues researchers are looking into to find possible genetic or otherwise causes of people who tend to sweat significantly more than others, with the absence of CF or other conditions that otherwise are linked with salty sweat.
Hope that helps, and happy running!