I just hooked up my tinyg to a zen toolworks, and wired the switches by the instructions here: http://www.synthetos.com/wiki/index.php?title=Projects:TinyG-Hardware-Info:. I think the pins for the Y limit switches and A limit switches are reversed. Steps to repro:
"$xsm=0", "$ysm=0", "$zsm=0", "$asm=0"
put a jumper across each pair of pins (one pair at a time), see that tinyg doesn't reset
"$xsm=2"
put a jumper across each pair of pins, see that it only resets on pins 3/4 and 5/6
"$xsm=0", "$zsm=2"
put a jumper across each pair of pins, see that it only resets on pins 11/12 and 13/14
"$zsm=0", "$ysm=2"
put a jumper across each pair of pins, see that it only resets on pins 15/16 and 17/18 (this is unexpected)
"$ysm=0", "$asm=2"
put a jumper across each pair of pins, see that it only resets on pins 7/8 and 9/10 (this is also unexpected)
"$$" output: http://pastebin.com/9eTfyMxw