Arborvitie (Eastern White Cedar) when used in the landscape have a few problems:
Low salt tolerance. Road salt is particularly obnoxious.
Winter sunburn. In spring they are getting a double dose of sunlight from direct sun, and reflection off the snow. This dehydrates the needles. But the roots are still frozen, so they can't replace the water. This is made worse if they are immediately south of a house or solid fence, as this creates a warm pocket, and adds another source of reflected sunlight.
Insect pests. In addition to spidermite, there is at least one scale insect that troubles them. If you feed your resident bird population in winter, they will help with this. We feed year round, and as a result have about a metric tonne of chickadees that spend their summers gleaning spydermites, and aphids off our trees.
The needles don't live forever, so brown needles and twigs in the interior of the tree are common. Trimming off the occasional external ones comes with ownership.
Thankfully, spider mites have many natural enemies that help reduce infestations and limit population overgrowth. Some of these natural predators include:
Phytoline P (Phytoseiulus Persimilis)
Amblyline cu CRS (Predatory Mite)
Anderline aa (Predatory Mite)
Exhibitline sf ( Predatory Thrips)
In many cases, the predators take care of entire infestations without the need for human intervention. Due to chemical spraying and the loss of beneficial insects, spider mites may have less predators to worry about in the area. However, the mites may also run rampant in greenhouses and interiorscapes where workers prefer not to use chemical pesticides. These areas have less natural predators, providing a safer breeding ground for mites to grow in numbers.