OED says that:
Middle English buryel, biriel, incorrectly formed as a singular of byriels, buriels n., q.v.; in later times associated with nouns in -al from French, such as espousal-s.
Etymonline.com says that:
"act of burying," late 13c.; earlier "tomb" (c. 1200), false singular from Old English byrgels "tomb," from byrgan "to bury" + suffix -els;
How is the word burial incorrectly formed? What would be the correct form if it wasn't incorrectly formed?
According to OED, buriels was used but it is obsolete now.
Here is the etymology of buriels from OED:
Old English byrgels strong masculine, cognate with Old Saxon burgisli neuter, < burg- ablaut-stem of bergan bergh n. to protect + suffix -isli- ; compare Old English græfels quarry, fætels purse, etc.