Prolific "water sprout" formation (sucker-like growths that form on branches or trunks as opposed to just the root system) suggests a tree is under stress or has been physically damaged. What the cause of stress is can be hard to determine after the fact, but wood-boring insects are a possibility. If enough sap flow into a branch is cut off by the damage of tunneling insects in the wood, then dieback will occur, though the prevalence of this can depend on the type of borer. Borers do tend to target trees (or do worse damage to trees) that were under stress at the time, such as from drought or overly-wet / poorly-drained soil. Dogwoods are not highly tolerant of either condition, for example, and if growing in full sun as opposed to partial shade, might have more vulnerability.

Branch dieback isn't normal per se on dogwood, though it can be commonplace due to the variety of ailments they can contract. You can use the information on the linked page to see if any symptoms match what you're seeing, though admittedly borers are hard to detect without looking inside removed, split-apart dead wood, or if they have not yet emerged as a generation of adults from the tree.

Given the age of this tree, you may want to have it evaluated by a certified arborist. (Some work for tree-care companies but others consult independently.) They cannot treat or cure every condition but can help with a diagnosis via an in-person inspection, plus can look for general sources of tree stress that might be able to be alleviated to improve its condition.

Trees usually don't need fertilizer, but unless you happened to grossly over-apply something (it doesn't sound like it) that caused root "burn" from excessive nutrients, then that was probably unrelated. Only a laboratory soil test can reveal if the soil is deficient in any particular key nutrients, plus measure the acidity (pH) to see if it's within the preferred range for certain species.  (Home test kits are not accurate or detailed enough.) Flowering Dogwood, for example, prefers acidic soils, and if the pH were too high, it could struggle to absorb what it needs. In that scenario, adding more nutrients wouldn't solve the problem; the pH needs to be lowered in order for it to access what is already in the soil. We can't enlarge the photo much but the leaves don't appear to be indicating a nutrient deficiency, nor is a deficiency a trigger for water sprouts or suckering.

Although our water sprout information page above suggests removal of young water sprouts, we defer to what an arborist suggests, since it would be helpful to determine why they appeared relatively suddenly in the first place. In the meantime, for now, just continue monitoring the tree for watering needs since we are behind on rainfall in many areas of central MD. If mulch is used around its base, make sure it's not mounded up against the bark (it should be pulled away a few inches from the trunk and only used in a flat layer about three inches or less thick).