was thinking about HDFC 856 level..imo the seller is most likely to be some institution taking their profits off, they absorbed most of the buyers around 856 and so when 856 broke there were not many buyers left and they couldnt keep up the buying, this was evident from price action as 2nd attempt around 1424 hr failed. This is different from when short term traders are shorting in which they have to get out when a significant level breaks and thus creating more up move.
This is valid even when a big trader is shorting futures as his stop wont be near and break of 856 wont be that much important for him as is to traders like us.