depends on how much soil you put over the roots---several inches likely not bad---foot or more especially of heavy soil might be harming the surface or "feeder" roots. assuming that there has been no other disturbance or cutting of the roots, or change in moisture or drainage, or other damage to the plant, the added soil MIGHT be the problem. the rule of thumb IMHO is don't mess around with tree roots surface or otherwise especially on big established trees if at all possible. what symptoms of "distress" are showing foliage changing color or dropping off the tree??? pictures might help determine the problem but better yet maybe talking to your county extension agent or a competent arborist (NOT tree surgeon) might get your better help. good luck.

If this is a coast redwood these are floodplain adapted, can have quite a layer of mud dumped on them in a short period of time and survive. Otherwise the main problem with filling under trees is it cuts off the water from fog drip and rain showers to the roots. If your tree began to look bad during the summer, dig down and see if the soil is moist under the fill.

well, it may depend on just what tree is really involved---some people use the term for the "coast redwood" (sequoia sempervirens) which IS tolerant of having the roots periodically buried but the "giant sequoia" (sequoiadendron giganteum) which is likely NOT so tolerant of excess soil being dumped on it's surface roots can also be called by the very same name which can be very confusing as the two trees while closely related have somewhat different cultivation requirements and environmental tolerances.