What are some causes of rust in stainless steel

Stainless steel resists corrosion by a process called passivation, which is similar to why aluminum doesn't “rust. “

The high chromium content diffuses to the surface, and reacts with the oxygen in the air, forming CrO, which is basically inert, like a ceramic. (in aluminum, Al2O3 forms, which is ceramic). This passive layer prevents further reaction, and therefore protects the underlying iron from forming iron oxide or “red rust.”

How can this fail?

If the oxide layer is scratched in an oxygen poor environment, the protection will be gone and the corrosives in the environment can cause rust or other corrosion. This is most prevalent in marine applications. Example are propellers on ships are typically a bronze alloy for corrosion resistance.

Galvanic corrosion. When dissimilar metals are in electrical contact, they form a galvanic cell, and the more reactive of the two metals corrode. Strictly speaking this isn't rust but it's corrosion. Stainless steel are fairly inert but they can be the victim of galvanic corrosion if care is not taken. (this is why in home construction copper gas or water lines are hung from joists with copper straps and never steel straps.)

Some chemicals are just awful and may attack stainless steel on its own right. Examples include automotive exhaust products. That's why exhaust systems are special grades of stainless steel to handle the chemical attack. This may sacrifice some resistance to normal rust, but the corrosives in auto exhaust are faster.

There are several other forms of corrosion that can break stainless such as crevice corrosion, stress corrosion cracking, hydrogen embrittlement,… But those are unique from rust.

Incidentally, the reason iron oxide (red rust) is bad is because it's less dense than the underlying steel. That means when it forms it expands, which leads to stresses, which causes it to flake off, eventually eating away the metal. Aluminum oxide doesn't do that.