Trying to figure out why this would happen. Was the pump still running when you disconnected the hose? Assuming the grids were in the tank, everything should have collected somewhere between the pump strainer and the filter. Then, once the pump was turned off (if you even did that - I don't even bother turning it off ever since using the vacuum plate), nothing should have come back in the pool. If the grids wouldn't be in, then it would recirculate whatever you vacuum right back into the return lines. What is your normal operating pressure and is there good flow coming through the returns? Any idea what sort of shape the grids are in? Any holes or tears in the grid material?
Good thought...but even then I can't imagine why stuff would flow back into the pool. I think it'd be worthwhile to drain the tank and then remove the push-pull valve (simply done by removing the three bolts that attach it). Then, check to see if the two o-rings are in good shape. If one or both of those is compromised, that could result in flow direction issues.
