It sounds like the combination of markers and the double increase. In order to have the decreases angle in, you want the central stitch of the three stitches involved in the decrease to be directly above the decrease two rows before it. This is pretty difficult when using markers, as they will get eaten into the decrease.
I wouldn't bother with the markers myself, as it's pretty easy to recognise the double decreases by sight. But if you want to use markers, put it in after the centre stitch of the decrease. When you get to two stitches before the marker, slip 1, k2tog (removing the marker - this will be a bit tricky), then put the marker back in after the decrease. This way the marker is always immediately after centre stitch of the decrease.
Or you can use two single decreases and put the marker in between them.