Hi Hub and welcome to GoY. Could you add a bit more info about yourself to your profile like where you live please. Also need more detail of how you were growing the laburnum and if it died suddenly or over a period of time.

also how old was the tree, they don't tend to be that long lived in my experience. Has there been any toadstools around the base? they are susceptible to Honey fungus attack.

Yes this could be the cause, i have noted on several times over the years where Laburnham have been lost to fungal attacks.