Or other personal events in the life of the owner. Likely the gap before being active again wasn’t anticipated, they kept thinking the horse would be active again soon, but that’s not how it worked out. Divorce. Serious financial problems. Long-term serious illness. Things of that nature.
Was it the same owner? Or possibly the original owner had the horse for years and then sold it.
It is also possible that the horse had a soundness problem that someone was trying to treat with rest, bring him back and things not going well, try something else … again, no expectation of the total time off. Just going from one month to the next, trying this and trying that thinking for a 30-day cure, then on to the next thing.
Whatever the reason, the horse gets back-burnered. Especially if the owner has their own land to keep him at pasture for a time.
This kind of pattern is a lot more frequently found in non-racing private owner horses – the owner has intentions but something is always in the way of fulfilling them.