The Digger first started to appear in the early 70s in the chopper world and it is often credited to be first built by Arlen Ness (back then Ness built cool bikes). The style that makes a chopper a digger is typically a long and low look. Other common this you will see are diamond and hexagon shaped gas tanks (which are usually long and long gas tanks as well), drag pipe exhaust, and large old school style drag wheels.
Notice the long and low look on this Arlen Ness digger.
To get the long and low look for the digger, they were usually built with low gooseneck frames and would have a big rake to the front fork. The frames usually have backbones that have been chopped so they can be lengthend for the long look.
Pink Taco has the longer frame to give it is digger look
Some people say the style was influenced by the drag bikes of era and they were meant to resemble dragbikes. The main story of it is Arlen Ness attended a drag bike event in the early 70′s where he saw a bunch of Harley sportsters. This give Arlen Ness an Idea, take a sportster, chop the backbone to lengthen the frame, put a longer rake on, add a diamond shape tank and add a 70s style drag tire.
Here he and she are inspired by drag bikes. Notice the Diamond gas tank.
The origin of why its named the digger has its own story as well. My guess of why it’s called the digger is because it’s based of a drag bike and back then dragsters were known as diggers so it is a digger stylebike. The other story I have heard is because of how low they are (though not as low as a lowrider) they looked like they were digging into the ground so they name digger was given.
Thanks to Arlen Ness diggers grow in popularity through out the 70s and become some of the bestchoppers ever built. Diggers still seem to have a strong influence on modern choppers today! They may have been on of the two influences for pro street choppers (the other was pro street hot rods).