Did it rain/really windy recently? I've had a few lychees outside before and noticed that it really does not take much at all for the leaflets to get twisted/malformed/die when odd weather comes in. If the leaves are even a few more days old they'll survive, but super fresh growth just can't take it at all.
It also might've decided it had enough new leaves given the roots it has to work with and killed them off. Either way I wouldn't worry, I've had younger lychees than that drop every single leaf, look dead for a month and a half, and then flush out again; so I think you'll be fine in the end.
pH problems can also cause that, but I'm not sure we can diagnose that without more information like soil pH and how wide of a change happened whenever you gave it sulfur, + I don't suspect it to be pH anyway. Overall I think its being dramatic. just my two cents