It could be an issue with your setup parameters.
If you are using Chilippr or CoolTerm, head to the console and enter $$ and return to dump your parameters. Copy and paste them to a text file on you favorite Cloud drive (Gdrive, OneDrive, etc) and post a link to them back here.
One item that looks strange in the above is that Homing Z should find the maximum, not minimum
Also be sure to review this wiki item:https://github.com/synthetos/TinyG/wiki/Homing-and-Limits-Description-and-Operation