why the big drop?, page-11

The rights issue is 21 rights for 40 main shares. eg not 1:1
This means 0.525 the cost (value?) of a main share.
The rights issue was for $28.29 per right. So the theoretical value of the share, relative to the right issue, is (1 / 0.525) x $28.29 = 1.90 x $28.29 = $53.88

Then throw in some external factors that alter what the market sees the share as truly worth from day to day. eg DOW, materials sector etc etc. Could be up, or down, value direction.

So if you owned RIO shares, you would want be getting $53-88 at least if you were 'shifting' to your rights you subscribed to.
The total number of rights is not equal to the total number of RIO shares, so only X amount of total RIO share owning people would do that sell. More (how many?) would likely KEEP the share and just ADD the rights to their portfolio.

Then there are the tons of RIO owners that did not subcscribe and thus have no great motivation to sell, outside of 'normal' reasons to trade them.

All in all, I don't see that there should be any "one to one" sort of corelation between the rights issue price and the share price - probably more like some offset ratio, seeing there are far less rights issued than total main shares. eg $28.29 rights issue means more like $58.00 main share price.
THEN whatever the normal external market/world factors that occur, move that price to.

RIO was high and curving off a crest anyway ($70's), so the falls (and shape) they are doing now is pretty well just a continuation of that process anyway, it seems.