Sources with direct knowledge tell us ...  Sam frequently complained about Britney getting physical with him during their 7 years together. We're told there have been numerous fights where security has had to step in, but in one instance there was no security in sight.

Our sources say there was one occasion where Sam says he was sleeping in their bed when Britney flew off the handle and began punching him. We're told Sam did not strike back, but was stunned as his wife pummeled him.

We're told that alleged incident went down early this year, right around the time Sam was photographed with a black eye and bite marks on his forearm.

Paparazzi noticed the bruising and tried to ask Sam about it, but he tried to play it off ... asking photogs not to shoot him.

Our sources say Sam was particularly concerned because Britney had a fascination with knives. We're told there were knives all over the house, including in their bedroom. Britney, as one source put it, "was paranoid someone was going to get her, and she needed the knives as protection." The sources add Britney tended to "fly off the handle" at the smallest slight, and that terrified Sam.

As we first told you, Sam and Britney got into a huge argument a few weeks ago when he accused her of cheating, and that's what triggered his divorce filing. But, we're told that was simply the straw that broke the camel's back.