Definition of thread
1 a :a filament, a group of filaments twisted together, or a filamentous length formed by spinning and twisting short textile fibers into a continuous strand
b :a piece of thread
2 a :any of various natural filaments the threads of a spiderweb
b :a slender stream (as of water)
c :a projecting helical rib (as in a fitting or on a pipe) by which parts can be screwed together :screw thread
3 :something continuous or drawn out: such as
a :a line of reasoning or train of thought that connects the parts in a sequence (as of ideas or events) lost the thread of the story
b :a continuing element a thread of melancholy marked all his writing
c :a series of electronic messages (as on a message board or social media website) following a single topic or in response to a single message
4 :a tenuous or feeble support hung on by a thread
5 threads plural :clothing
"All Threads by date" is one of the Forum categories. It looks to me definition 4 best covers why forum postings are called threads.
