This was answered in another thread.
Quick answer: Go to your Sequencer Still Setup --> Open your settings preset --> Turn on Anti-Aliasing --> Under "Advanced" set the "Engine Warmup Count" to 300.
It’s because when you click render, the engine resets the scene and takes a screenshot from that point. This means that the particles haven’t had time to spawn in and move around. Once the engine warm up is set to 300, when you render an image the camera will wait 300 frames before rendering. This gives enough time for the particles to spawn in.