Blame it on the book: Baseball Haiku: American and Japanese Haiku and Senryu on Baseball, edited with translations by Cor van den Heuvel and Nanae Tamura.

Samples:

Intentional walk --
Each fan winding up
his own boo

A blimp
above the baseball stadium
floats by the moon

A spring breeze
flutters the notice
for baseball tryouts

dog days of summer
twenty-three games
out of first

Okay, despite the haiku in the title, if you've been counting you know these are not haiku. They're all senryu:
Wikipedia:

Senryū (川柳, literally 'river willow') is a Japanese form of short poetry similar to haiku in construction: three lines with 17 or fewer "on" (not syllables) in total. However, senryū tend to be about human foibles while haiku tend to be about nature, and senryū are often cynical or darkly humorous while haiku are serious. Senryū do not need to include a kigo, or season word, like haiku.

I can imagine the argument at the publisher's -- "We have to call them haiku, nobody in America knows what senryu means." You'll notice that word doesn't even appear on the cover image, despite the "official title."

But since a real haiku is 17 syllables, 5-7-5, here's mine:

Long ball approaches
the pole, where is Carlton Fisk --
We need him again

Now there's a haiku in the bunch. I figure Carlton Fisk is the telltale kigo.

Go ahead, write your own in the comments section.

To get you warmed up, here's another baseball haiku page.

There's a kicker: Jack Kerouac wrote a baseball senryu:

Empty baseball field
— A robin,
Hops along the bench

Listen to him read it.