The most common reason for an action like this is brand protection, and it is believed that Leatherman is of the opinion that the folks at TTC are using the Leatherman brand in unauthorized ways, reulting in the order. Further speculation includes quality and warranty issues, both of which can affect how the Leatherman brand is represented, and affects tool costs.
Texas Tool Crafters (TTC) has modified tools with alternate blade materials and various coatings on components to create higher end tools out of standard Leatherman tools. They do work wth other brands as well, however until now Leatherman products appear to have been the majority of their product line.
Recently Victorinox sent a number of these types of letters to members of the modding community for their use of the Victorinox name and logo, and it is important to note that these companies are not "beating on the little guy." Like any company, they have to protect their brands, their interests, and by extension their employees, and this often starts with ensuring that their products are represented properly in the market. Warranty claims on tools that have been modified are a cost that these companies should not be responsible for, however they then risk upsetting (and possibly losing) the customer with the broken tool, and represents a no-win situation for the manufacturer who has done nothing wrong.
It is a bit surprising that Leatherman has waited this long to enforce the legitimate use of their brand, however there has been speculation both on our forum and on Facebook that Leatherman has a new legal team, and they are interested in making a name for themselves. Whether this is a reality or merely some upset ranting we cannot confirm, however we support Leatherman's attempt to protect their brand, and we hope that TTC can find a way to continue their operation in a manner that satisfies all parties.
Let us know what you think in our forum.