You are seeing the classic disadvantage of (mis-)using an ANOVA when your independent variable is ordinal. ANOVA is meant for situations when the independent variable is categorical (e.g. political party or fifth grade teacher). It has no notion that there is an ordering between years and thus can only detect the case when one year shows a significant deviation from others, not the case when there is a gradual trend over the years. If such a trend is clear when looking at the years in order, but not when looking at any two given years, the ANOVA is likely to miss it.