Summary
Hangovers make Rose Weasley grumpy. Scorpius Malfoy makes Rose Weasley grumpy. Waking up in places other than her bed makes Rose Weasley grumpy. So it's completely understandable that combining all three makes for an incredibly grumpy Rose Weasley, and a smart person would avoid antagonising her at all costs. Turns out Scorpius Malfoy isn't very smart.