You urethra is basically a slit so it comes out flat but surface tension makes the ends come out and different speeds which makes your piss spin making it stay together. If you look close your piss will be in a straight spinning stream.
When something gets stuck in the slit (like if you recently jerked off) it messes up the flow.
Other things that can effect it are urethral polyps and some infections (swollen prostate can cause stop start of stream)
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