My female is in season and today my male mounted her and did his business while she willingly stood for him. They tied for about a few seconds and she freaked out. I tried to calm her down but me trying to approach her made it worse and within seconds they detached. Why is this? Is there a chance she could become pregnant from this? This has happened twice today and once yesterday.

It's not at all uncommon for bitches to freak out once tied. You need two people if it is anything but a very small breed -one to hold the bitch and one to hold the dog, once tied. You need to stay very close so you can get hold of her quickly and easily. The bitch breaking away can cause serious injury both to herself and the dog so it must NOT happen. And yes, she could very well be pregnant.

My champion bitch though a total slapper hated the tie and managed to turn turtle on us despite being held. She tried to do this at every mating and had to be held firmly after the dog turned. She had two litters then missed twice.

You do not need a tie for pregnancy, but a tie does prove that ejaculation has occurred, where without  you can't be sure, and the tie does help the sperm to get up where it's needed.

Even the briefest tie means she could be pregnant. Without you'd certainly want to have seen the ejaculatory phase of ' ladder climbing' from the male.

It's not necessary to have a slip tie much as breeders prefer to see one.   One of my boys never achieved a tie and (much as I didn't allow him to be used much!) only missed to one bitch who came back on her next season, to my other male.   Successful mating again, but again no puppies.   Both were proven males.  She was taken to another male not in my kennel, and..... failed to conceive.   Clearly not the fault of the males, slip tie or not.

In my breed, the bitch normally has to be supported across the knee of the owner/handler during a mating which means no chance of a struggle but it's vital the handler holds both parties, to avoid the risk of injury.   Maiden bitches can freak out.   I had one of mine who was ready, but when penetrated, yelled blue murder.   She was the same with each delivery too.   I only took the one litter from her - and in truth, only needed one to provide me with a super puppy for the next generation (two actually, we kept a male and a bitch).

A 'real' tie is when the males penis is trapped inside the female through the swelling of the females internal tract.
A 'slip' tie is when this does not happen and he falls out without being held within the bitch....both can result in puppies.

As others have said you need 2 people and you need to be right there holding and steadying them. Hetty screamed the place down while the tie was starting and everything was swelling up, which made me panic a bit about what the noise levels would be at whelping, but luckily she was absolutely fine, just a whine or two.