Yes, this is a VERY irritating problem. I recently got a trackpad for my Mac, and found it nearly impossible to select sections of text - it kept bringing a dictionary instead of selecting the text! This would be enough to rule out using an otherwise wonderful trackpad for normal computer use, if there were not a solution. Seriously, what was Apple thinking?
Significant1's response gives clues to help solve the problem. The trackpad settings are different now so the suggestion does not work directly, but of course 6 years have passed so that's understandable. With a modern MacOS and trackpad, the most relevant setting you can select is called "Look up & data detectors." By default it is set to "force click with one finger." Apparently that is the problem: when you are selecting text you have to hold one finger on a corner of the trackpad while moving another finger to select the text, and it is very hard to avoid pushing too hard. Note - setting the click strength to medium or firm does not help. But you CAN set the "Look up & data detectors" to "Tap with three fingers." That appears to be sufficient to avoid undesired triggering of this (rather useless) feature when you are just trying to select text using two fingers.
I have not yet tried Significant1's terminal comment to disable the dictionary. I will keep that in mind in case the problem reappears.
Thank you!