Hi Charles
Still not sure what the question is, you mention the "trigger" and then wonder why other trees don't behave like maples. Two separate questions, seems to me.

In late winter and early spring, if the maple is "tapped," that is, bored with an auger, sap will flow from the wound in consider- able quantity, the flow being dependent very largely upon the temperature. Some other trees that bleed in the same way as the maple, though usually not to so great an extent, are Juglans cinerea, Cladrastis, Nyssa, and sometimes Prunus serotina. Later in the spring, just before vernation, the birch and grape bleed profusely, and, to a lesser extent, also Ostrya, Hicoria, Alnus, Malus, Crataegus, Salix, Ulmus, and perhaps a few other trees. 

Investigation of these various trees has shown that they fall into two very definite groups as regards the bleeding phenomena, with a different source, and perhaps different cause, for the bleeding in the two cases. Those which, like the maple, bleed early in the spring and are dependent upon temperature, constitute one class, while the late-bleeding ones, like birch and grape, which are not intimately dependent upon the temperature, constitute a class by themselves.

According to Clark, a mercurial gauge attached to the end of a frozen branch of sugar maple indicated pressure and suction when the temperature was raised and lowered precisely as it would have done upon a maple tree during the ordinary alternations of day and night in the spring of the year -when the sap is flowing

The best sap days are those in which a bright sunny morning with rising temperature follows a frosty night. The flow is greatest early in the morning, decreases gradually as the day advances, and ceases altogether during cold nights. It is not a daily perio- dicity, however, since on many days no sap flows, while again the flow may continue all night It seems necessary that the rising temperature should cross the O0 C. line in order that there should be a good "run" of sap. If the temperature remains for several days above this point or for several days below it, the flow will rapidly diminish, and in from 24 to 36 hours cease altogether

A rise of only a few degrees will often cause very great pressure if the rise passes the zero point Centigrade. On the other hand there may be considerable fluctuation in temperature without great fluctuation in tension. This happens when the temperature does not cross the zero line. The pressure fluctuations are greatest early in the season.

There seems to be an undoubted relation between the occurrence of pressure in the maple and the fluctuations in temperature. 
In connection with no other fluctuating factor of the environment can such relation be shown.  
Pressure exists only when the temperature is rising. When it falls or remains constant, suction occurs.

Wiegand, K. M. (1906). Pressure and flow of sap in the maple. The American Naturalist, 40(474), 409-453.

Karl McKay Wiegand was one of the foremost authorities on taxonomy in the world.  His leadership of the Department of Botany at Cornell University for nearly 30 years earned the department the highest reputation. 

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