The seasonal dynamics of vegetation canopies are critical indicators of ecosystem responses to climate change. While much research has focused on trends in overall greenness, the drivers governing the seasonal rates of canopy growth and senescence remain poorly understood. This study investigates the dynamics of canopy development and senescence on China's Loess Plateau (CLP) from 2000 to 2018. We used monthly NDVI differences (VNDVI) to quantify canopy change rates and employed partial correlation with climatic data and solar-induced chlorophyll fluorescence (SIF) to identify their drivers. Our results reveal a strong seasonal compensation effect: accelerated canopy growth in the early growing season (interannual VNDVI trends ranging from trends of 0.62 to 1.05 x 10-3 month-1 year-1), predominantly concentrated in semi-humid zone, was largely offset by a mid-season slowdown. Photosynthetic activity (proxied by SIF) was the most influential factor, explaining canopy dynamics across over 50% of the CLP. However, its coupling with canopy growth showed a strong seasonal dependence, shifting from strongly positive in spring to negative in autumn. This decoupling suggests a fundamental transition from a source-limited to a sink-limited state, where plants prioritize carbon allocation to storage rather than new leaf growth in the late season. Climatic drivers also showed strong spatial heterogeneity; precipitation was critical in arid western areas, while temperature and radiation were more influential in the semi-humid east. These findings highlight that internal sink limitations, not just photosynthetic capacity, can constrain vegetation growth. This seasonal vulnerability of CLP's carbon sink has critical implications for managing this large-scale revegetation area under future climate change.