Jenkins always chooses a slave by alphabetical order
By default, Jenkins will try to use the node that the same job has run on previously/last.
This is on the assumption that the last run of the job (and therefore the last node it ran on) has the latest SCM checkout, thus reducing the number of files required for update, and therefore speeding up the build.
To change this behaviour, use various plugins, like the aforementioned Scoring Load Balancer plugin.