ISP controlled modem counts your incoming data in a short rolling window. If it exceeds a value, it throttles below your normal speed and buffers or drops incoming packets to slow things down, until the bandwidth budget is compensated for (and the dropped packets are all counted, so it could be stuck there longer than anticipated), then switches to normal speed until traffic goes below a threshold, then switches back to full speed burst. Worst case the flow control reacts slower than the modem burst switching and you end up in an oscillating pattern.

The trick is just to do your own rate limiting without burst at the right limit, that bypasses whatever shitty burst implementation your ISP modem has and weird shit won't happen anymore.