Well, experience and the Wiki says it's often related to fighting multiple opponents. Maybe enemy entities trapped behind fortifications along bottlenecks would count for nearby dwarves and make them more prone to enter trances?
So, this has probably been researched extensively already, but when fighting one-on-four, all with heavy armour and high Dodge so they don't kill each other quickly, the outnumbered dwarves entered trances with similar timing, so there's a countdown rather than or along with chance. After falling unconscious and passing the time limit that seems to restrict Trances, they wake up and immediately go into trance. Trances seem to last for less than half a day, and during one versus three testing the countdown between them seems to be between four and five days, leaning closer to five. Oh, and I'm obviously doing this in Object Testing mode, so there are no personalities involved. I'll get ten or more dates for a good enough average and then check if number of opponents affects the countdown or duration somehow, and what exactly counts as being outnumbered towards Trances.
So, this is what I've got so far: One-on-one most definitely hasn't triggered any trances, nor have two-on-one, while three-on-one, three on-two, three-on-three etc. all delivered. 1-on-1-on-1-on-1 also triggered trances, but less often for some reason (I have a few ideas, but they're pointless to share until I've tested them), while 1-on-1-on-1 didn't work. Three seems to be the magic number. Proximity is important, as just a few steps away they stop counting towards the required three. I also tried placing nine Team #1 dwarves on pillars two z-levels up, but they didn't seem to count along with the ground level #1 towards making the #2 go into trance. I need to do more testing with enemies only one z-level up.
This is the timetable for one dwarf fighting against three other dwarves, first date is the start, second the end: 4th-4th, 9th-10th, 14th-14th, 18th-18th, 23d-23d, 28th-28th, 4th-4th, 9th-9th, 15th-15th, 19th-20th, 23d-23d, 28th-28th. After that, the three in Team #2 started falling unconscious as well, and the dates became irregular: 3d-3d, 10th-10th, and then the lone #1 died and that's that. I think the area was a bit too big, because when the #2s began hardnapping they were separated for short periods. So I don't know if unconscious opponents count towards the Martial Trance Trinity. I'll do some more testing tomorrow, when I'm not on the verge of spontaneously falling asleep.
Oh yeah, skills don't seem to affect the regularity of trances. They went from Proficient Dodgers to Legendary in several skills without any particular fluctuation in their martial countdown. I'll try vampires tomorrow, since they're so much stronger and can thus quickly show if higher attributes have an effect.