Why are there tension forces in bolts in a fin plate connection?
In most cases, fin plates use one plate to connect the member. The consequence of this is, that we apply small eccentricity to the connection. This eccentricity will cause additional prying forces on plates, which applies also to bolts. To demonstrate this effect, please try to make double fin plates to see no tensile force or make plates stiffer and the force should be smaller.