What caused it? Most bizarre.
Caused by a glut of greenfly aphids the previous year (the cheif food item for ladybirds, bless 'em), the ladybird breeding pattern changed due to that glut - such that next year - 76 - their were far more ladybirds as there was expected to be far more aphids - but in the event there wasn't..
I remember Derby Pool covered in them.
After a while they got pretty cranky with little or no food and were known to bite - which they do not do normally...