momentum/particle physics -A2 Physics

When pions are used to treat brain tumours they are slowed by the tissue in the brain and cause little damage. When a pion is moving very slowly it may be absorbed by the nucleus of an atom. The atom nucleus then becomes unstable and breaks up into
several fragments.
Explain why these fragments shoot out in all directions. (3)

The answer mentions about conservation of momentum, how it was 0 before and it fragmented to conserve momentum...

[momentum (1)
Zero / negligible momentum before (1)
To conserve momentum (fragments go in all
directions) (1)]

How do you know the momentum was 0 before?
And how does it splitting into fragments mean that the momentum is 0 after too and that momentum is conserved?

Because momentum is a vector and its direction must be taken into account.
A car of mass 1000kg moving to the left at 10m/s has momentum 1000 x 10 = 10,000 kgm/s
A similar car moving to the right at the same speed has equal but opposite momentum.
If the 1st car is + 10,000kgm/s then the 2nd car is - 10,000kgm/s

If an object at rest splits into two equal parts and both parts fly in opposite directions, the initial momentum was zero because the object was at rest. After the split the objects, like the cars, have equal and opposite momentum.

The total is still zero because the one is plus and the other minus. They add to produce zero.

but how do you knowwww it was zero before? it doesn't state it was at rest...?
it says it was unstable, how does this help answer the question? [if it does]
thankyou

They are talking about the atoms in tissue your brain. I think you can safely assume these atoms are either at rest or have "negligible" momentum. They use the word negligible in the MS.
The point is, after the nucleus has burst into fragments, the fragments go in all directions and the momentum of all the fragments added together is zero or very nearly zero.