According to the readme on autolabor, it automagically does that stuff for you. I can see that it may be trying to diversify the work pool or maybe those dwarvs were just nearer to objectives. This is however the risk you take when using an optimization tool out of the box (the risk being that results may vary amongst users).
Anyway, the readme also states a good way to prevent autolabor kicking in, which is to burrow dwarves. It also recommends to check help autolabor for extended usage, to which I'm pasting the output here.
[DFHack]# help autolabor
autolabor: Automatically manage dwarf labors.
Usage:
autolabor enable
autolabor disable
Enables or disables the plugin.
autolabor <labor> <minimum> [<maximum>]
Set number of dwarves assigned to a labor.
autolabor <labor> haulers
Set a labor to be handled by hauler dwarves.
autolabor <labor> disable
Turn off autolabor for a specific labor.
autolabor <labor> reset
Return a labor to the default handling.
autolabor reset-all
Return all labors to the default handling.
autolabor list
List current status of all labors.
autolabor status
Show basic status information.
Function:
When enabled, autolabor periodically checks your dwarves and enables or
disables labors. It tries to keep as many dwarves as possible busy but
also tries to have dwarves specialize in specific skills.
Warning: autolabor will override any manual changes you make to labors
while it is enabled.
Examples:
autolabor MINE 2
Keep at least 2 dwarves with mining enabled.
autolabor CUT_GEM 1 1
Keep exactly 1 dwarf with gemcutting enabled.
autolabor FEED_WATER_CIVILIANS haulers
Have haulers feed and water wounded dwarves.
autolabor CUTWOOD disable
Turn off autolabor for wood cutting.
With some customization, it is possible to configure autolabor to restrict labors to a certain quantity of dwarves or to disable it for certain labors. The labor tokens seem to be those used by the game, which are available at the wiki.
I personally use DwarfTherapist to assign labors, as I like to control which specific dwarves do which labors.