A bisporus makes a good point:why go to the trouble of digging twenty big holes (or one huge one) when an axe and some avgas would do the job without effort?And I agree there is a lot of photographic evidence of the latter method of disposing of planes, there are none of them being buried.
However, these planes were not being disposed of in a hurry because an airfield was being over-run or left behind.So there was plenty of time.There was plenty of labour (personnel and foreign labourers would be available).The planes were new and expensive:£12,604 each,plus (Estonian order for 12 Spitfires in 1939, according to wiki) which would be about £3 million each today.
There may well have been the hope they could be recovered, so it may have been seen as worthwhile burying them, in these particular circumstances.