Answer:
I believe, one of the men had killed a woman and left her in the barn, making Lennie run because he knew he did something wrong.
Step-by-step explanation:
The reason why I say this is because Lennie was a bigger man, and suffocated the woman when she went to make a noise, causing Lennie to freak out and cover her mouth, which ended up making her suffocate.
Hope this helps.