It's suspected that someone is shooting out the windows using pellet guns, or perhaps throwing rocks.
It appears the buses are being targeted but it's not clear if they are being attacked by someone on the side of the road or inside a moving vehicle, Montiel said.
There is also a general resentment in San Francisco towards tech companies for driving up rents regardless, and attacking the shuttles may be the outlet for this anger. One incident is said to have went down just outside of the city of Woodside, and another outside of Los Altos Hills.
Montiel warned that the projectiles are considered risky as they could possibly distract the bus driver and cause a collision.
"Due to recent incidents of broken windows along the commute route, specifically on highway 280, we're re-routing coaches for the time being", Apple wrote in a Tuesday email to employees.
Mashable obtained an internal email at Apple explaining that several buses would be rerouted for employee safety reasons, adding significant time to the workers' commutes. No injuries were reported.
While the exact method by which the buses are being attacked remains unclear, that fact that real damage has been done is not. "It's not like they are hidden", he told The Guardian. Four years ago, Google buses were attacked in protests against gentrification and increasing living costs amplified by the presence of tech companies.