Yeah, this is moisture in the bowl and it shows up more at the end of a smoke because there isn't as much tobacco left to absorb the moisture. This moisture can come from the tobacco being moist from the tin, or from tobacco that produces a lot of moisture burning, or from a smoker who lends more moisture in the form of saliva as he/she smokes, and sometimes from a pipe the design of which tends to accumulate moisture in some low point in the bowl. Also, clenching a pipe while walking, driving, or doing some activity may increase the amount of saliva you add to the moisture factor. The moisture may be a combination of two or more of these. I am by nature a fairly dry smoker, but I occasionally get a little sizzle or wheeze going at the end of a smoke, and this is helped by gentle and judicious tamping, and maybe drying out the blend overnight before smoking it, just one or two bowls.