Ditto to Beau - esp if he's obese, the horse has very likely been laminitic(& if it's systemic rather than purely mechanical, it won't be only in 3 feet) long term, but it has only been *recognised* when it reached the 'final straw' point of the horse becoming obviously lame from it. Unfortunately that is exceedingly common, as many(even vets & farriers) still don't recognise 'sub clinical' laminitis. It is very rare for a truly 'suddenly, out of the blue' 'founder episode'. Many horses are 'always been fine' but perhaps 'ouchy on hard ground' or 'bit tender after the farrier' or 'need shoes' for eg, until 'suddenly, without warning' they're seriously lame & declared 'foundered'.
Chronic 'good condition', obesity & insulin resistance is the common cause of systemic laminitis, but not the only one. It can be due to drugs, stress, mechanics, poisoning... anything that causes inflammation & damage of the laminae of the hoof. 'Spring grass' is a commonly blamed culprit. However, IME, spring grass & the extra sugars in it(or autumn grass, or at other times) are generally more of a 'final straw' for horses that due to chronic metabolic probs are overly sensitive to(like 'junk food' to a type 2 diabetic). Feeding grain/'sweet feeds' is a common cause too, which can be due to IR type 'lifestyle diseases', but can also be due to acidosis/gut upsets from the grain. Racehorses for eg. commonly suffer laminitis.... albeit 'sub clinical', which often doesn't(obviously at least) effect their performance in the short period they're racehorses.
One more thing to consider - people often think of fresh grass as 'dangerous' or 'high sugar', while hay is 'safe'. You even still hear advice from vets to that end, for 'laminitis prone' horses - don't let him have any grass, but feed him hay only. Worse still, feed the horse on straw or oaten chaff only(because this is considered an 'empty filler'). Unfortunately, grass produces sugars from photosynthesis, uses those sugars only in active growth over night, and doesn't lose much of them when dead/dried/baled. Therefore, hay may have just as high sugar content as the grass it was grown from, and straw/chaff, coming from cereal grass(namely oaten) tends to be quite high sugar content(and that's even if there are no grains in it!). So, depends how/when the grass is growing/was cut, the weather, stage of growth, the variety etc, as to how high sugar it is, but not so much on whether it's fresh grass or hay. Therefore, it pays to be careful - not just in spring - of what grass/hay our horses get, and especially if they're already obese/IR, I'd be very cautious to avoid high sugar, 'improved' varieties, such as ryegrass. If feeding hay to a 'high risk' beastie, if you think it's high or aren't sure, you can soak & drain the hay in clean water before feeding, which will leach out some of the sugar.