[EvaH (OP)]

James wants to know:

How did a piece of Mars rock break off to get to Earth for scientists to examine?

What do you think?

[Bored chemist]

As part of their "ongoing commitment to public transport" my local  bus company has rerouted the bus from here into the town centre.
The route is now so tortuous that I suspect the martian rock could have got on somewhere along the way.

[evan_au]

Astrophysicists expect that an asteroid strike on Mars could blast a piece of Mars rock into orbit around the Sun.
- Some of these random orbits intersect with Earth's orbit, bringing a piece of Mars here

A similar thing happens with asteroid strikes on the Moon - but because the Moon is much closer, the number of Moon meteorites on Earth exceeds the number of Mars meteorites on Earth.

Ironically, the latest NASA rover on Mars is taking back a piece of a Mars meteorite back to Mars, as a calibration sample.
See: https://mars.nasa.gov/mars2020/mission/overview/