In his book, "With Each & Every Breath", under the category "Visions & Uncanny Phenomena", regarding "Common Problems" during meditation, TB writes:

During meditation, "...if you sense that you are outside your body, you may feel tempted to travel around a bit on the ASTRAL PLANE, but you should resist the temptation. There are dangers there, and meanwhile you're leaving your body unprotected. You can get back into the body by calling to mind the four basic properties that make up the sense of your body as felt from within: the breat energy, warmth, coolness, and solidity."

I've never even heard a "traditional" Buddhist teacher even MENTION or VALIDATE the "ASTRAL", or talk about a "YOU" that can leave the body !!

I'm not a specialist on this but have projected out from my physical body a few times. My late teacher was excellent in this art, though, and have heard a lot of the topic from her.

If this has been said solely in the context of meditation, then of course, astral projection is a deviation from the practice. However, in general it isn't that dangerous. Also the question of leaving the body "unprotected" is a bit weird. Do we need to protect our body when we are in it? From what? Why it would be unprotected when we are out of it?

Does he explain why he says "there are dangers there"?

He is speaking in conventional terms. When he talks about you leaving your body and you getting back to it, he is not talking about the theory of selflessness but disintegrating and deintegrating the three bodies (physical, energetic/mind and awareness), especially the first two.

Glad to see traditionalists use nontraditional terms.

Wow !!! Thanks for the great responses !!   If Thanissaro Bhikkhu gives advice to meditators on how to deal with the "astral", maybe HE should give us ALL some advice on how to handle the "astral" if we CAN'T get back INTO our bodies....like....IF WE SUDDENLY DIE !!!  Most of us are NOT advanced enough spiritually to NOT be SHOCKED and SURPRISED when the moment of death occurs. Or, do the "masters" not even know, themselves ?

Did he explain the dangers?

Go read it yourself -- you just might learn something about the Theravada perspective...
http://www.dhammatalks.org/Archive/Writings/EachAndEveryBreath_v130123.pdf

No, he did not. That quote I gave is the whole section, except at the end of the last word, "solidity", he adds "(see the discussion of the Fourth Jhana, in Part Four)"....However, under the heading "Visions & Other Uncanny Phenomena", under which my posted segment appeared, TB started off:

"Sometimes they (visions, etc.) convey true information, sometimes false. The true information is especially dangerous, because it leads you to trust whatever  pops into your mind, so that you start falling for the things that are false.....can lead to strong conceit, as you begin to feel you're somehow special. This pulls you far off the path."

He did not explain any more about "leaving the body unprotected" or other "astral dangers".  Thank You all for your responses !!

Superstition brings bad luck ;-) 

OK.