Why were the members of the DA so easily captured by the inquisitorial squad?

Question

In Harry Potter and the Order of the Phoenix, Harry attempts to use Umbridge's fire to contact Sirius after having his final vision during OWLs (Ordinary Wizarding Level exams; magical equivalent of high school diplomas I believe). When he's yanked back out he finds that five members of the DA ('Dumbledore's Army', commonly refereed to as DA in the books) have been captured and restrained by an equal number of Inquisitorial Squad members. It's made clear that the DA members have been roughed up, but no mention is made of any harm to the Inquisitorial Squad members.

I would concede that any duel could be won by circumstance and luck as much as skill. Still, it seems terribly unlikely that five separate duels are all lost by students who have spent the entire year preparing specifically for this eventuality. Further, so, that the winners should come out of it completely unscathed. This builds on the oddness of the earlier scene when the Inquisitorial Squad captures DA members fleeing from their final meeting, without any mention made of resistance (not even a shield charm).

While Umbridge may have given a slight advantage to the Inquisitorial Squad, she has been shown time and again to be incompetent, both in terms of general magical knowledge and in practical magical abilities. So it seems unlikely that she turned the tide so completely herself. And the DA members were engaged in an illegal activity and specifically on guard against Inquisitorial Squad members and Umbridge, making surprise unlikely. And it has been established that several members of the Inquisitorial Squad are not magically competent.

So how were the DA members so easily overpowered if, as the book claims earlier, they really were highly competent at defense against the dark arts? What is really going on here, that the DA members continually lose with minimal resistance to fellow students?

Answer 1

They were taken by physical force and/or simply surprised.

If they had gotten involved in a physical fight while they were unprepared they would have probably lost.

There was silence in the office except for the fidgetings and scufflings resultant from the Slytherins' efforts to keep Ron and the others under control. Ron's lip was bleeding onto Umbridge's carpet as he struggled against Warrington's half nelson.
_{^{- Harry Potter and the Order of the Phoenix, Chapter 32 (Out of the fire)}}

To me this quote shows that Ron was punched or elbowed or something of the like in the face, and then was subdued by Warrington I cant think of any spells that would give someone a cut on the lip, but they may be out there, also most of the Slytherins are larger (by far) than the D.A so it wouldn't have been much of a fight if they had been jumped. Since the slytherins were never mentioned having cuts or injuries it is more probable that the D.A were surprised.

Later they do fight with magic and the D.A wins.

"How did you get away?" asked Harry in amazement taking his wand from Ron.
"Couple of Stunners, a Disarming Charm, Neville brought off a really nice little Impediment Jinx," said Ron airily, now handing back Hermione's wand too. "But Ginny was best, she got Malfoy - Bat-Bogey Hex - it was superb, his whole face was covered in the great flapping things.
_{^{- Harry Potter and the Order of the Phoneix, Chapter 33 (Fight and Flight)}}

So once it was a fair fight with Magic the D.A win, this leads me to believe if they were actually fighting with magic at first the Inquisitorial Squad had surprised the D.A and that's how they won.

Answer 2

Harry was in an extremely vulnerable position (head unhearing/seeing, butt in the air if I understand it correctly) and doing the most important work of the group. A fight there could potentially harm Harry while he's vulnerable as well as render him unable to get the information he needs. Mitigating factors also include the fact that they were ambushed, and that Dumbledore's Army had not yet engaged in open war with Umbridge and the Inquisitors, and to do so would cross a big red line.

So in the interest of getting the information, not using up their one chance to attack Umbridge when the DA was woefully unprepared, and keeping Harry safe, they surrender to reduce tensions.