Stress rings, grass rings - call them what you will. They are often thought as normal or natural and casually dismissed. Because they are so common few 'professionals' remark on them, let alone give advice, so its hardly surprising that the uninitiated pay them scant attention.
But Event Lines should be taken seriously, observed, noted and the effort made to link back to the 'event' that caused them. This is where a diary or great memory help, because an Event Line will typically first show about 1cm below the coronary band roughly 4-6 weeks after the 'event'. This of course will vary between horses.
All sorts of things can cause an Event Line. Typically a horse is/has received an overload of sugar/starch. But it could be a bad reaction to worming or an injection.
Grace has had some substantial Event Lines as you can see in the picture below. Grace has had (probably previously undiagnosed) sub clinical laminitis. She is sensitive to sugar, but also to vaccination.
In December she had her second of three set up innoculations and suffered a bad allergic reaction. Sure enough, as her foot has grown down, this 'event' has shown up as a substantial Event Line on her hooves.
Event Lines can be rings or grooves. At the time of writing I know of no definitive evidence as to what influences ring or groove. I just know that some horses tend to throw rings, some grooves and some a bit of both.
Don't ignore them, they are vital feedback direct from the hoof that something has happened to upset your horse's metabolism.
From observation only I am concerned that it is possible that horse shoes mask the symptoms of an event, but it doesn't mean a horse is not struggling metabolically, just that the feedback mechanism is compromised.