When this happens to you, it is a bug - please report it as such and we will work to get them all taken care of. The name of the mob in question, and your quest (if any) are useful in tracking these down. Bonus if you can tell us the name of the ability that knocked you out.
So let me get this straight; ANY time in PvE where we get knocked out while entrench is up; we should report it as a bug?
I know for sure the imperial quest on Alderan where you have to slow the assault on the outposts and get the droid chips (which you return to house Thul and find an attack under ground afterwards), the commandos fire rockets at you that make you bounce out of cover, even with entrench on.