Ann: Investor Update Presentation & Confirmation of FY18 Guidance, page-292

"..but its a one-off, and it doesn't effect the long term value of the business"

Well... maybe not of the business, but it can have a profound and long term effect on the share price! It's way easier to drop out of an index than climb back into it.

As you pointed out some funds will have to reduce their exposure to Vocus once they're out of the ASX100 because of the fund charter, risk strategy, etc. The key thing here is that those funds cannot buy back in until after Vocus re-enters the index. So the time line is this:
1) Price of VOC falls too low for ASX100 index.
2) Funds reduce exposure pushing VOC lower.
3) For VOC to recover it must climb by the combined 1) and 2) without the participation of those funds.

Did somebody say short positions were increasing ( @krugerrands , I'm looking at you )?
There's your reason.

My prediction (which I rarely get right) is below $2.50 again before the end of January.