Most placements are made at a slight discount to the market price, in order to make them attractive to sophisticated investors. This means that the placement should theoretically result in a reduction in the price, the fall being greater, the greater the discount at which the placement is made.
The reasoning here is simple. If the shares were worth, say, $5 yesterday and a placement is made at a discount to $5 overnight, today the average share will be worth less than $5. Part of the capitalization of the company has been redistributed from the existing shareholders to the recipients of the placement. This is why the areholders who do not participate in the placement are wont to object to the placement and why in some large placements, companies also offer some new shares to existing shareholders.
The reason companies get away with this form of wealth redistribution is that the placement is seen as being advantageous to all the shareholders in the longer term. There can be some value in this argument. In a simple case where the costs of a rights issue would have been greater than the discount offered in the placement, the benefit to all is quite clear. However, in other situations the strategic value perceived from the placement can actually cause the share price to rise rather than fall after the placement is made.