应该是 供给增加->价格上升->spread下降

书上70页:
Prospective new supply induces many traders, analysts, and investors to advo- cate a defensive stance toward the overall corporate market as well as toward individual sectors and issuers. Yet the premise, “supply will hurt spreads,” which may apply to an individual issuer, does not generally hold up for the entire credit market. Credit spreads are determined by many factors; supply, although important, represents one of many determinants. During most years, increases in issuance (most notably during the first quarter of each year) are associated with market-spread contraction and strong relative returns for credit debt. In contrast, sharp supply declines are accompanied frequently by spread expansion and a major fall in both relative and absolute returns for credit securities.
(Level III 2012 Volume 4 Fixed Income and Equity Portfolio Management, 5th Edition. Pearson Learning Solutions p. 69).
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thanks, bridget.

it clarify this point.  so actually improving market conditions drive the supply.