Virtually all of it is my guess (and it is just a guess).
Quite often, when the market is delivered news that it "already knows" the headline can turn into a sell on the news event. The markets expect 50 basis points, if that is the announced rate cut then a jump higher in reaction to the headline could turn out to be a short-term (intraday) event which doesn't see much in the way of followthrough to the upside. A 75 basis point rate cut is not something which has been discounted by the markets and that would be the kind of news that could start a stampede of bulls (higher prices).
Technically, the trend remains for higher prices but this leg higher (Nasdaq) is losing momentum. With the Fed on an easing path, though downside remains limited. (In other words, I don't know what's going to happen on Wednesday and my technical measurements on the market aren't really tilted in one direction or the other. That's the way it is sometimes.)
The Nasdaq has virtually fulfilled expectations for a close in the 2840-2870 area before the FOMC's announcement on Wednesday Jan. 31. Tuesday's attempt to move higher ran into selling right in the middle of the double focus of resistance mentioned in Monday's comment: 2847-2862 (Tuesday's high print was 2861.71).
Intraday measures of price and volume for the Nasdaq offered no clues as to potential market action on Wednesday.
Immediate (intraday) support for the Nasdaq is a thin shelf 2827-2804 with a focus 2809-2804. More substantial short-term support is 2794-2763.
The Nasdaq is testing a broad band of resistance (based on end of day data) in the 2814-2916 area. Within this band of resistance is a focus of resistance 2847-2862 if the index moves above this level during Wednesday's market then 2862-2847 becomes support.
The S&P 500 has immediate support in the 1368-1353 area with a focus 1365-1361. Supports are stacked with the next support in the 1350-1339 area.
The S&P 500 has a substantial layer of resistance in the 1351-1389 area. The next resistance is 1414-1439. Cherney is market analyst for Standard & Poor's