Somewhere in the old threads I think, but I'm in a rush and can't look it up right now. Quick points:

• Quirrell organized the whole Dementor visit from the beginning
• Quirrell waited until Hermione was running up to the Dementor and would've seen it herself anyway to "suddenly notice" Harry's wand next to the Dementor's cage
• Quirrell suggested the Headmaster leave early, before it was Harry's turn
• Quirrell instructed Harry's friends to "give him space" even though he probably knew that it was better for Harry to be surrounded by his friends as Dumbledore said
• Quirrell undermines Harry's confidence just before he goes in front of the Dementor (If you can't do it, I at least will understand)

All plausibly deniable, and exactly Quirrell's style.