Are you certain that you used too much neem oil or are you attributing the leaf drop to the neem oil application simply because spraying the tree preceded the leaf drop? If, at the time you applied the neem oil, you were following the directions and felt like you'd applied an appropriate amount, my guess would be that your application of neem has nothing to do with the leaf drop. I did not apply any chemicals, but my BRTs have also been dropping leaves. My experience has been that's just something they do around this time of year when they're indoors for the winter. I'm not sure if the cause is environmental (perhaps the very low humidity in the house) or if it's something that originates from the tree's circadian rhythms (i.e. the plant's internal clock is set to drop last year's leaves at this time of year to prepare for the next growth cycle). One of my BRTs has dropped almost all of its leaves but I can see plenty of healthy swelling buds that are getting ready to put out new growth.