Why were Anabaptists called Anabaptists? Anabaptism, according to the OED, means "a second baptism, re-baptism", but the answer to this question says that Anabaptists only baptize once. Why were Anabaptists called Anabaptists, then?
Anabaptists do not consider themselves to be 'rebaptizers'. Their view is that baptism is only valid if it is done as a result of a personal faith and particularly they consider that baptisms of people too young to understand what is happening are invalid. Their members are required to be baptized as adults (or at least to be 'of an an age of understanding'). 

The start of the Anabaptist movement was at a time when virtually everyone would have been baptized as an infant. Anabaptists would require members to be baptized 'again' (as an adult), not because they believe two baptisms are required but because they believe the first (as an infant) was invalid. Similar situations happen fairly frequently today.

The name 'anabaptist' (rebaptizers) was applied to the group by detractors, who would have considered infant baptism valid and seen the Anabaptists as baptizing 'a second time'. Members of the groups came to accept the name, and use it to this day.

See: http://www.christianitytoday.com/history/issues/issue-28/1525-anabaptist-movement-begins.html