What causes bike to slip out of gear?

My CG sometimes slips out of gear from second. It either slips down into neutral just after changing up from first, or in the worst case scenario it slips from second to first when riding along(which makes me sheet myself because the backwheel locks up).
I thought that maybe it was because of the way i was shifting, but this is something that has only happened over the last 500 miles and my style hasnt changed.
It's a little concerning since the slip down into first makes the bike extremely unstable.

Thanks for any help...

Global warming

i done this this morning. At a point of gearing you can change gear without using a clutch so if your foot is resting a bit heavy on the lever it will drop down.

Check oil level + chain adjustment

Originally Posted by Magnum

My CG sometimes slips out of gear from second. It either slips down into neutral just after changing up from first, or in the worst case scenario it slips from second to first when riding along(which makes me sheet myself because the backwheel locks up).
I thought that maybe it was because of the way i was shifting, but this is something that has only happened over the last 500 miles and my style hasnt changed.
It's a little concerning since the slip down into first makes the bike extremely unstable.

Thanks for any help...

When i had a CG as temp transport, mine developed a fault where it would not change up mainly 2nd on the 1st attempt.
That was due to a worn dog lever or whatever. There was a modified replacement for it. The bike was an N reg and i had it from new so wasn't abused.