Stress due to environmental changes, acclimation shock, lack of food and high nitrate levels (above 10ppm) are some of the biggest causes for tuxedo urchins not surviving. Once they start dropping spines, they are also prone to bacterial infections, from which it is hard to recover.
Like all invertebrates, tuxedo urchins need to be drip acclimated as inverts are very sensitive to changes in water parameters. It has been said that longer acclimation times are needed but a lot of reefers have been successful in acclimating urchins for the same amount of time as other invertebrates. BUT, they have ensured to match the salinity of the water and slowly allowing them to acclimate to the new aquarium’s temperature.
Be careful of stirring up the sand in your tank as disturbing it could cause a poor water quality due to detritus etc. entering the water column. Also, if doing water changes, make sure water is added slowly, over a few days, and not just dumped in. This will ensure that the sand bed isn’t disturbed. This also goes for bare bottomed tanks. Detritus settles under rockwork and is easily released into the water column.