There are hundreds of reasons, it be caused by currents, bad direction, strong wind.
Ships make a wake that draws them closer to each other, so if they are sailing close to each other they will eventually collide.
solids
When two tectonic plates collide with each other. If they collide, they usually slide under each other, causing a tsunami. However, If the collide head on, ground is pushed upward, creating a volcano'mountain
It creates mountians
Yes
armageddon
No, gas particles can touch each other when they collide.
they hug each other
collide into each other
By definition, if they are parallel to each other, they will never collide, so the chance is zero.
convergent boundaries collide but divergent boundaries move away from each other.
The warm and the cold air collide violently with each other