The term "trailing leg" refers to the leg at the back when someone is walking or running. It is the leg that follows behind and is generally not as active as the leading leg. Full definition
It appeared as if there had been some contact, but referee Mike Jones felt that Ki had initiated it by trailing his leg into Koscielny and booked the South Korean for diving. (itv.com)
Purce was legitimately taken down, but the ref decided D'Angelo getting to the ball first was enough to excuse taking Midge out with her trailing leg. (sbnation.com)
Kane held out his trailing leg in order to make contact with Gabriel, and referee Michael Oliver was tricked into awarding a penalty. (caughtoffside.com)