Nothing stops it being called a theory. There is a lot of nonsense talked about the word "theory"...
It starts with "Darwin's theory of natural selection". People who wanted to ban it from schools claimed "It's only a theory, not science fact. Because it's a theory, we shouldn't teach it." People who wanted to teach it said, "Theory doesn't mean that. Theory means a 'well-substantiated explanation of some aspect of the natural world'. So we should teach Darwin's theory."
They then went on to make a point of "what is a theory" and test students on their ability to remember the definition. But in actual use there are many senses of "theory", from "unproven but testable idea" to "the non-practical aspect of the subject (music theory)"
"Theory" in the definintion in your question is being defined to capture the meaning of "theory" in "Theory of Evolution" and "Theory of Relativity". These are both organised systems of knowledge (there are many aspects to these theories but the parts work together. In relativity the parts are "spacetime", "constant speed of light", "Lorentz transformations" ,"equivalance of gravity to acceleration" E=mc² etc etc.)
You can call your tripping example a "theory". But it's a bit too small to be a "scientific theory" in the way that the theory of relativity is a theory. It's not an organised system of accepted knowledge. It's just too trivial.