my mums miniature schnauzer has always been friendly with people and other dogs, she does bark at strangers, and is known to chase runners and people on bikes but only barks at them she is not at all vicious (she goes straight on the lead if we see any of these).  But we have become concerned the other day when we were walking her and the other mini schnauzer on there usual field walk, a man walked by and she ran and began jumping at the back of his legs and barking non stop, the man threatened to kick her and began shouting which made her worse as he thought she was vicious, we put her back on the lead and have not let her off since.  This is out of character she has never ran at a person before like that, we see lots of people on our walk but we are worried to let her off if she will continue to do this, my sister had walked her the day before this incident and she was like it then as well.  Also she attacked my nans dog just for walking by her when she had her lead on (never done that before either) any one know why this is happening?

It would be hard to guess without seeing the behaviour, but any change in behaviour like this should be checked out by a vet incase it's down to a medical reason.