In the same manner that crank revolutions will loosen the left pedal, the rotation will also loosen the bottom bracket at the drive side. English-threaded bottom brackets account for this by having left-hand thread on the drive side, the "fixed cup". On the non-drive side, the "adjustable cup" is right-hand thread, and is used to adjust the bottom bracket bearing play (on the old-school style bottom bracket - obviously not on the newer cartidge-style bb).
French, Swiss and Italian bottom bracket cups are right-hand thread on both sides. Some versions of these had a lockring on both cups, but most did not.
Once, during the time when I was first beginning to do fixed-gear conversions and sell them, I bumped into a young man by the name of Matt, who had an interesting bike. It was a 1980s Bianchi road frame, copper-plated and converted to fixed-gear. As he was telling me about it, I was crouched down checking out the crank and chainring on the bike.
"Say," I asked, "did you build this up, yourself?"
"No," he replied. "The guys at Shop X put it together."
I always ask that, first, before pointing a problem with a build. I don't want to embarass someone.
"Well," I pointed out, "you might want to take it back and have them reinstall the bottom bracket. It's in backward."
The Bianchi had an Italian bottom bracket, and it had been installed with the adjustable cup and lockring behind the chainring.
"Oh," he said. "I wondered why it was so hard to adjust the bottom bracket."
A couple of months later, I got an email from Matt, asking about a bike I had for sale. After I answered his question, I asked him if he had gotten his bb reinstalled.
"No," he replied, "they told me that that bottom bracket was made that way. So, they sold me one that went in right."
I refer to this shop as Shop X simply because I wasn't actually there when this happened. For all I know, Matt might have been embarassed and just bought another bottom bracket to cover for himself. But, in light of other dealings I've had with these clowns, I don't think so.
I think that the owner and mechanic were probably drinking beer and/or smoking dope (as usual, for them) when they built Matt's bike up. They installed the bottom bracket backward, and just didn't want to admit thier mistake.
So, they took advantage of a gullible kid and sold him a replacement bottom bracket, rather than turning his existing bb around for him. I hate shops like that. You may recall that similar treatment from another shop, toward me, resulting in me becoming a mechanic so that I woudn't be at their mercy.
Left-hand thread...On top of its other advantages in a fixed cup, it pretty much makes you install the bottom bracket in the correct orientation. Usually.
But, that's another story for another day.