Stock price reactions (to new security offers)

Empirical findings [Smith (1986)]:

1. average abnormal returns are non-positive,
2. abnormal returns associated with announcements of common stock sales are negative and larger in absolute value than those observed with preferred stock or debt,
3. abnormal returns associated with announcements of convertible securities are negative and larger in absolute value than those for corresponding non-convertible securities,
4. abnormal returns associated with sales of securities by industrials are negative and larger in absolute value than those for utilities.

Explanations?