I noticed that contracts end in June, but job fairs are in February and other times not so convenient.
When a teacher is told that his contract will not be renewed in June, then I guess that means he will be unemployed for a while.
Fire in June, Hire in February
Typically you would know well in advance that your contract is not being renewed so that you can recruit. Most schools (not all) want staff retention and thus pay a lot of money to attend fairs so they will meet top candidates. Because of this schools usually ask staff their plans as early as October. International schools must deal with housing and visa issues and this takes time. Most top schools like to have it all sewn up by Feb or March to allow their HR to process paperwork and ensure everything will go smoothly in August. It is also a non critical time (no major testing going on) so teachers an administrators can spare the time without having a major impact on students. Once you are used to this schedule it all makes sense, but looking in I guess one could see it as inconveinient. I personally like knowing I have a job in April so I can fully devote my mind to student revision for exams.