Just like the title says

A poor seal.  Too much grease.  If there is no air pocket in the hub, when the grease gets warm it expands.  Since there is no, or too little air in the assembly, it will force its way past what was a good seal.  

 

Another possibility is that when a warm/hot wheel hits the cooler water, a partial vacuum may be formed which sucks in water.  One nice thing about bearing buddies is they have a spring loaded diaphragm which allows the grease to expand, provided it is not over filled compressing the spring when the grease is cool, then when it expands the grease may force its way past the seal.

 

I've been guilty of over filling the hub, ending up with grease splattered all over the wheel.  I was lucky that it forced its way past the seal without blowing it out.  Cleaned up the mess and drove to my destination (on the Road Trip) and home with no more loss of grease.

I am no professional, but I have had my share of self induced trailer problems over the years.

generally if it is coming out the back you blew a seal. if it is coming out the front the bearing is overheating.

 

On my first trailer one of the hubs started leaking out the front. I did not think much of it and just made sure it always had grease. one day while going down the interstate, I smoked the bearing and ruined the spindle. first thing the shop asked is, was it leaking grease out the front. Lesson learned I guess.

 

my next trailer had bearing buddies. I figured you just fill it with grease until the springs bottom out. a couple trips later one of the hubs started blowing grease out the back. I took it to the shop, because i did not want to smoke another bearing.  they told me I overfilled it and blew out the back seal.

 

fortunately, my current trailer has sealed bearings, so I can't do much damage.

Do you have bearing buddies? Mine always comes out of the tiny overflow hole that lets you know when it’s full. Sometimes it could come from around the dust cap cover. I have pulled a boat all over the southeast with grease slinging everywhere, at least that means you still have some grease in there! I like my outboard to be smoking and my hubs to be slinging hahaa. Well I would rather they didn’t sling grease but it doesn’t lead to problems (for me) as long as it’s slinging from the outside as opposed to the inside 

As mentioned, coming out the back is a bad seal, coming out the front, it's too much grease.  Only put enough grease to cause the fitting cup to lift and float a little, no more.  A set of bra's help keep it contained. 

Another recommended action, if you tow very far, when you get to the ramp, give them enough to float the fitting cup.  That way when it hit's the cold water and all the warm grease contracts, it won't pull water in.

If you grease them cold, the friction heat causes expansion and greatly increase the problem of pushing grease out.

10 hours ago, Way2slow said:

As mentioned, coming out the back is a bad seal, coming out the front, it's too much grease.  Only put enough grease to cause the fitting cup to lift and float a little, no more.  A set of bra's help keep it contained. 

Another recommended action, if you tow very far, when you get to the ramp, give them enough to float the fitting cup.  That way when it hit's the cold water and all the warm grease contracts, it won't pull water in.

If you grease them cold, the friction heat causes expansion and greatly increase the problem of pushing grease out.

you learn something everyday. thanks