This happens because when columns = p (prime), numbers in each column share the same remainder mod p, creating visible diagonal patterns as multiples of p are eliminated from primality.
Not so much that cols is prime as that cols+1 or cols-1 has lots of factors - see for example 25 or 91 or 119. But it does seem like numbers adjacent to primes have a lot of factors.
The more factors an (even) number n has, the more likely it is that n+-1 is prime because those numbers cannot have any of the factors of n as factors. At the same time it is impossible that n+-2 or 4 are prime and unlikely that n+-3 is prime because 3 is likely to be a factor of n. And if additionally 5 is a factor, the primeless gap is even wider. So the primes stand out.
When the col is seven, there are a lot of diagonals going from top right to bottom left. When col is five, from top left to bottom right. Are runs of consecutive sexy primes also this frequent for larger numbers, or does that pattern break down at some point?
I find the patterns from cols % 30 == 0 very interesting (30,60,90,120, etc.) .. just straight vertical lines.
And if you go up or down by one (119 or 121) they appear to "rotate" left or right.
Very cool viz tool.
Almost all of these patterns that you see don't really come from primes. If you display numbers not divisible by first 100 natural numbers you get pretty much the same picture.