It's muddy, ya know? Ever gotten your foot stuck in deep mud? 
I got stuck in quicksand once.
Not hard to get out if you're barefoot, although I lost my shoes

My mom stole some random guys broom to pull my bro and his friend out. I think out of the four of us that were stuck, only one person came out with shoes, and they only had one

Maybe having speed in muddy water being tied to mech foot size/complexity could be a feature (JK, it would basically be a greifing tool against all clan mechs, with only mechs that had small Atlas like legs winning out. aka none)