last month I took my bike to my LBS to get the cog/lockring tightened because I felt some slippage when backpedaling. Now just recently I felt a big slip pedaling FORWARD, few minutes later it slipped again but this time backpedaling.

I rotafixed it (upon doing so I noticed the cog was really loose) and then tightened the lockring.

My question, why did it slip forward? I understand that when you pedal forward the cog gets tighter, then this must have been the lockring that slipped and gotten loose? but why was the cog so loose when I rotafixed it (I was able to tighten it quite a bit). if pedaling forward would tighten the cog then why did it slip forward? i'm confused.

and sorry if what I'm describing isn't making any sense. I'm new to all this.

I'm riding a raleigh rush hour.

[edit- i should mention i rode it hard today and didn't have any more slipping problems, could this happen again? I don't want this $@%# happening in traffic.

thx