What are some causes of high salinity in ocean waters?

There are processes that increase salinity of the oceans and factors that decrease salinity.

Evaporation causes the salinity of the ocean to increase. This is the primary process taking fresh water out of the oceans. Fresh water enters the ocean through rain and rivers. Rivers bring in a small amount of new salt and dissolved minerals that remains after evaporation.

Freezing also affects salinity. Near the poles, particularly the antarctic, when ocean ice freezes it eliminate a significant amount of salt from the ice, leaving a saltier and denser brine behind. This denser antarctic brine sinks and initiates a current the flow from the antarctic water northward towards the deeper ocean. This is an important influence on circulation in the southern oceans.

The increase salinity of the oceans is also counteracted by processes in the oceans sediments that remove salt so on the average there has been little change in ocean salinity over the last few billion years.

Note: Ocean salt, as described here, is not merely sodium chloride, but also has a fraction of other dissolved ions. See related questions.