Rotorspeed - it is very easy to end up IMC with no warning, particularly at night. You simply do not see clouds.
In my situation you're correct, it was ludicrous what happened because we had fantastic visibility. The main problem was that in failing light I underestimated the ceiling height from our point of take off and within 5 or 10 seconds from take off was full IMC. Poor light plays tricks. Trust me, lesson learned.
Although some training is good to get an appreciation of flying under instruments until you have actually flown IMC you have no idea what it is like. I remember that once I had completed my instrument training as far as I was concerned I could have flown through a hurricane. It was not until I actually flew with an experienced pilot IMC that I realised that it is totally different. At that point I reralised a huge lesson: DON'T GET YOURSELF IN THE SITUATION IN THE FIRST PLACE.
To all those taking instrument lessons good luck, but trust me on this one: be under no illusion that you are skilled enough to be able to fly IMC.
You'll only get out of it alive if you're lucky.